Electrical wiring inspection system

ABSTRACT

A system for testing and documenting the electrical wiring in a building, for example, includes a Portable Circuit Analyzer (PCA) that is connected to the building&#39;s Load Center through an umbilical cord. The PCA is in wireless communication with a hand-held computer device, such as a personal digital assistant (PDA) as now widely available, provided with custom software according to the invention. The electrician connects the PCA in succession to each circuit in the building, operating each switch, and each fixture or appliance, while recording the test results of the circuit element on the PDA. The PCA measures the resistance and length of each circuit thus established. When the test process is completed, the PDA is enabled to generate a complete schematic diagram of the building, including, for example, an identification of the branch circuit to which each fixture, outlet, appliance, or other load or connection point is connected.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from provisional application Ser. No.60/555,331, filed Mar. 23, 2004.

FIELD OF THE INVENTION

This application relates to electrical test equipment for testing theintegrity of electrical power distribution systems and for locatingfaults in the same.

Essentially all aspects of the present invention except theElectromagnetic Wire Locating method disclosed in Section 7 weredisclosed in the Provisional Application listed.

BACKGROUND OF THE INVENTION

Residential dwellings, office buildings and businesses in the UnitedStates are typically wired in accordance with the National ElectricalCode, inspected after initial construction, and then never again testedunless a problem occurs. Yet, after 10, 20, or 30 years of continuoususe, wire insulation dries out and cracks, contacts become loose, lightsockets degrade from heat, switches lose their spring, and numerousother aging processes take their toll. Additions and modifications tothe electrical wiring over the years, either by certified electriciansor perhaps a handyman, or even the owner herself add another level ofuncertainty about the wiring. The difficulty is that these changes overtime may eventually lead to a fire hazard, a shock hazard, unexpectedfailures and costly repairs, or perhaps all of these things.

Degraded or loose contacts and overloaded circuits can lead to contactarcing, a process that starts intermittently but given enough time maybecome persistent. Electrical arcs even at low currents can developtemperatures that exceed the ignition temperature of most commonflammable materials and therefore pose a significant fire hazard.Prudently, in response to the electrical problems of the past, theNational Electrical Code has been developed to mitigate this danger. Forexample, the code specifies that all wiring connections be contained innon-flammable junction boxes thereby reducing the probability thatarcing at a connection will come into contact with flammable materials.Nevertheless, connections sometimes degrade over time and begin arcing,causing flickering lights, circuits to go out, and sometimes a fire. TheUS National Fire Protection Association estimates there were 38,300residential fires of electrical origin in 1998 resulting in 284 deathsand 1184 injuries and $668.8M in direct property damage (from NFPA'sU.S. Home Product Report, Appliances and Equipment Involved in Fires,January 2002). According to the NFPA “Electrical distribution equipment(i.e., wiring, switches, outlets, cords and plugs, fuse and circuitbreaker boxes, lighting fixtures and lamps) was the third leading causeof home fires and the second leading cause of fire deaths in the UnitedStates between 1994 and 1998”. But even when arcing does not result in afire, it can cause electrical interference, flickering lights,intermittent service and eventually substantial damage to the arcingcontacts themselves as well as flammable materials in the vicinity.

Aging wiring and/or incorrect repairs can also lead to wiring faultsthat present a shock hazard for building occupants. The loss ofgrounding protection, for example, particularly in wet areas, canpresent the danger of electrocution. The US Consumer Product SafetyCommission estimates that of 230 electrocution deaths involving consumerproducts in the United States in 1995, 23% resulted from installed homewiring.

In addition to the danger implicit in developing electrical faults, thecost of waiting until faults develop into a noticeable problem can behigh. Arcing on loose or dirty contacts of a circuit breaker, forexample, will eventually lead to the need to replace the entire LoadCenter.

And finally, the electrical wiring in older buildings is often not welldocumented. The label on the Load Center of an older building is oftenbarely legible or in error. The process of correcting the label, i.e.,determining which circuit breakers control which circuits, can be quitetime consuming, often requiring more time than is warranted absent aproblem. Without adequate documentation, diagnosing problems is moredifficult and branch circuits may be inadvertently overloaded.

A number of tools and instruments are commercially available to test anddiagnose various aspects of household wiring. Inexpensive plug-inmodules, for example, are commonly available to test whether a groundedsocket is wired properly. Plug-in Ground-Loop Testers have also beenmarketed over the years to test the current-handling capacity of groundreturn paths. Common electrical test equipment, such as hand-heldvoltmeters and the like, and more sophisticated instruments such asoscilloscopes can also be used to test home electrical wiring; however,these tools yield only limited information. A thorough test of all thewiring in a home done manually would likely take a couple ofelectricians many days to complete and would therefore not beeconomically feasible.

There exists a need therefore for an instrument and method to quicklyand economically test the entire electrical wiring system in a home,office, or business, so as to identify and locate dangerous conditionsand flaws, and thereby reduce the chance of electrical fires and toprotect occupants from the danger of electrical shock. A less importantadvantage of such testing would be to accurately document the system inthe process.

TERMINOLOGY

One purpose of the SafeWire™ system is to test electrical wiring inhomes and other structures for faults and degradation due to aging andas such has useful application in testing older wiringinstallations—installations that used fuses instead of circuit breakersand knob and tube wiring instead of Romex-type cables. Also, someelectrical components are commonly known by multiple names. So to makeclear and unambiguous this application and the attached claims a briefexplanation of the terminology used herein follows.

The term “Load Center” in this application refers to the principalwiring distribution and protection point of an electrical wiring system.It is also commonly referred to as the “Service Panel”, “Breaker Panel”or “Panelboard”. In larger systems, an apartment building for example,there may be a single large electrical panel, typically known as the“service panel”, and then a sub-panel for each apartment, the sub-panelin this context being referred to as a Load Center. Modern Load Centerstypically contain a Main Circuit Breaker to protect the Load Center anda number of Branch Circuit breakers for branch circuit protection. OlderLoad Centers employ fuses instead of circuit breakers so to avoid theneed to call out both terms at every reference, and thereby make thisapplication more readable, the term “circuit breaker” shall includefuses of all types.

The term “branch circuit” refers to the circuit conductors between abranch circuit breaker and an outlet, light socket or directly-connectedload that it feeds. A modern branch circuit is typically either aRomex-type cable or individual wires in a metal conduit the latter beingtypical in businesses. Branch circuits in homes over 50 years of age maybe individual wires routed using a “knob and post” or “knob and tube”wiring scheme. Some early homes even made use of existing gaslightpiping to route wires in. Thus, the term “branch circuit” shall includeall of these wiring schemes.

The phrase “electrical power distribution system” in this applicationrefers to all the components used to distribute electrical powerthroughout a structure. For example, in a home, it refers to the LoadCenter including all the circuit breakers or fuses, all the wiring, theoutlets, switches, light sockets and anything else either permanently orsemi-permanently installed to distribute electrical power.Semi-permanently installed wiring may include, for example, extensioncords, outlet strips and the like.

The term “line voltage” refers to the standard voltage present on anelectrical power distribution system and the informal term “Hot” refersto a conductor that has the line voltage on it.

SUMMARY OF THE INVENTION

The present invention is a modular diagnostic instrument, referred to bythe inventor as the SafeWire™ system, that enables an individualelectrician to test every electrical wire, every connection, everyoutlet, every switch, every light, and every appliance in a house,typically in a few hours or less. In use, the electrician first attachesa novel device to the Load Center and then, moving from room to roomthroughout a house, plugs cords into outlets, flips switches On and Off,unscrews light bulbs, turns appliances On and Off and entersdescriptions into a hand-held Palm Pilot. The SafeWire™ system does therest.

Briefly, the SafeWire™ system comprises a Portable Circuit Analyzer(PCA) that is connected to the building's Load Center through anumbilical cord. The PCA is in wireless communication with a hand-heldcomputer device, such as a personal digital assistant (PDA) as nowwidely available, provided with custom software according to theinvention. The electrician connects the PCA in succession to eachcircuit in the building, operating each switch, and each fixture orappliance, while recording the test results of the circuit element onthe PDA. The PCA measures the resistance and length of each circuit thusestablished. When the test process is completed, the PDA is enabled togenerate a complete schematic diagram of the building, including, forexample, an identification of the branch circuit to which each fixture,outlet, appliance, or other load or connection point is connected.

For simplicity and clarity the SafeWire™ system is described hereinrelative to testing single-family residential homes. It applies just aswell, however, to duplex homes, trailer homes, apartment buildings,office buildings, factories, libraries, museums, and the like, all ofwhich employ electrical distribution systems that are substantiallysimilar to residential electrical distribution systems. Indeed,SafeWire™ testing according to the invention applies to any structurethat incorporates wiring to distribute electrical power. For example, itmay be readily adapted for use on shipboard, aircraft, large vehicles,and other vehicles. It may be used to test outdoor lighting systems,temporary special event wiring for musical concerts or trade shows andother specialized applications. The means and methods disclosed are alsoshown on standard single-phase wiring, but with only minor changes couldbe used to test 3-phase power distribution systems. It is therefore tobe understood that the disclosure of the use of the SafeWire™ system totest residential wiring in this application is provided specifically forthe purpose of providing the simplest possible explanation of a complexsystem, and that the SafeWire™ system may be useful for testing a widevariety of other electrical wiring systems.

The SafeWire™ system is also described herein relative to wiring thatconforms to electrical standards in the United States of America againonly to simplify the presentation. Indeed SafeWire™ testing may provemore valuable elsewhere in the world where a combination of higher linevoltages, older structures and perhaps less rigorous electricalstandards may make faulty electrical wiring more dangerous.

BRIEF DESCRIPTION OF THE DRAWINGS Drawings are Arranged in Accordancewith the Sections that Reference them

1. Safe Wire™ System

FIG. 1 is a simplified physical diagram of one preferred embodiment ofthe SafeWire™ System according to the invention.

FIG. 2 is a simplified physical diagram of an alternate embodiment ofthe SafeWire™ System.

FIG. 3 is a sketch of an Adapter Cable employed in the preferredembodiment.

FIG. 4 is a sketch of a Portable Circuit Analyzer (PCA) employed in thealternate embodiment

FIG. 5 is a drawing of a SafeWire™ Load Center Interface (LCI) moduleprovided according to the invention, attached to a typical Load Center.

FIG. 6 is a drawing of a SafeWire™ Micro-Energy Dielectric Tester (μEDT)module provided according to the invention, attached to a typical LoadCenter.

2. Safe Wire™ Portable Circuit Analyzer

FIG. 7 is a simplified schematic diagram of one preferred embodiment ofthe SafeWire™ Portable Circuit Analyzer (PCA).

FIG. 8 is a simplified schematic diagram of an alternate embodiment ofthe SafeWire™ PCA.

FIG. 9 is a simplified schematic diagram of circuitry for implementingan AC Balance Method, provided according to the invention, as applied atboth an Outlet and the Load Center.

3. SafeWire™ Load Center Interface

FIG. 10 is a simplified schematic diagram showing the measurementmethods used by the SafeWire™ Load Center Interface (LCI).

FIG. 11 is a simplified schematic diagram of an Input Module of the LCI.

FIG. 12 is a simplified schematic diagram of the voltage measuringcircuits in the Input Module.

FIG. 13 is a schematic diagram of the Power Input Cable.

FIG. 14 is a simplified schematic diagram of a Main Module of the LCI.

4. SafeWire™ Magnetic Probe

FIG. 15 shows an illustration of a Magnetic Probe.

FIG. 16 shows one Magnetic Probe attached to one pole of a two-polecircuit breaker and another Magnetic Probe just removed from the secondpole.

FIG. 17 shows one Magnetic Probe attached to one pole of a main circuitbreaker and another Magnetic Probe just removed from the second pole.

FIG. 18 shows a Magnetic Probe attached to a Grounding bus bar.

FIG. 19 shows an expanded cross-sectional view of one embodiment of theMagnetic Probe.

FIG. 20 shows an assembled cross-sectional view of the Magnetic Probe ofFIG. 19.

FIG. 21 shows an assembled cross-sectional view of the preferredembodiment of the Magnetic Probe.

5. AC Balance Method

FIG. 22 shows a stand-alone tester that uses the AC Balance Method ofthe present invention.

FIG. 23 shows a graph of typical Line Voltage versus Time measurements.

FIG. 24 shows a graph of Line Voltage versus Time with a rectified loadapplied.

FIG. 25 shows a stand-alone tester on an ungrounded outlet.

FIG. 26 shows a tester that uses the AC Balance Method with a separateGround reference.

6. Least-Time Propagation (LTP) Method

FIG. 27 shows an LTP measurement circuit on a typical 2-wire BranchCircuit.

FIG. 28 shows the timing waveforms of the circuit of FIG. 27.

FIG. 29 shows an LTP measurement circuit on a typical 3-wire BranchCircuit.

FIG. 30 shows an LTP measurement circuit connected between two outlets.

7. Electromagnetic Wire Locating (EWL) Method

FIG. 31 is a simplified schematic diagram showing a modified PCAconnected to a residential outlet.

FIG. 32 is the simplified schematic of FIG. 31, connected for testingtwo outlets.

FIG. 33 is the simplified schematic of the EWL tool adapted for locatingunpowered wires.

FIG. 34 is a simplified block diagram of the circuit of the EWL tool.

FIG. 35 is a graph of the output signal of one axis of the magneticfield sensor.

FIG. 36 is a simplified physical illustration of the EWL tool in use.

8. Software and System Operation

FIG. 37 is an illustration of the top-level PDA screen and variousTester screens.

FIG. 38 is an illustration of the top-level PDA screen and the Location,Outlet and Light screens.

FIG. 39 is an illustration of the top-level PDA screen and Switches andCircuit Breakers screens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Overview of the Safe Wire™System

The preferred embodiment of the SafeWire™ System of the invention isshown in simplified physical form in FIG. 1. The system is made up offour basic components that work together to enable a single electricianto take extensive measurements on a household wiring system in aconvenient manner and in a relatively short period of time.

An off-the-shelf handheld computer 1, commonly known as a PersonalDigital Assistant (PDA), provides the computing power and a userinterface for the SafeWire™ system. Custom SafeWire™ software runs inthe PDA and the electrician uses it to control and review all aspects ofthe SafeWire™ system. In the preferred embodiment of the presentinvention, the PDA 1 is one of several commercially-available PDAs withintegral radio communication capabilities, such as Bluetooth, towirelessly communicate with the other SafeWire™ components. Provision ofwireless communication (as opposed to a wired connection) enables theelectrician to hold the PDA in one hand, communicating with all theother elements of the system, while turning light switches On and Offwith the other hand, for example. As will be seen below, this enableseach component of the wiring system to be conveniently tested, and theresult of the test simultaneously recorded.

The second basic component of the SafeWire™ system is the PortableCircuit Analyzer or PCA shown as 2 in FIG. 1. The PCA 2 is carried bythe electrician throughout the house to perform testing on outlets,light circuit, light fixtures, appliances and more while household poweris on. A test cord 3, typically on the order of 25 feet long, plugs intothe PCA 2. A connector 4 at the end of the test cord 3 is disposed toaccept any of several adapter cables 5 via mating connectors 5 a. Theseadapter cables 5 allow the PCA 2 to be readily connected to any one ofseveral household outlet types 7 via matching plugs 6 (one physicalembodiment of the adapter cable 5, this one for a standard outlet, isshown in FIG. 3). All outlet types found in both new and olderresidential wiring systems, including 220 volt types, are preferablysupported by provision of the corresponding adapter cables 5, only asampling of which are shown in FIG. 1. A spring-loaded lamp socket plug8 is also provided to allow the PCA to connect to a lamp socket 9 byremoving the bulb 10 and then inserting the plug 8. As discussed indetail below, the PCA 2 (in combination with the other components of thesystem) measures the resistance of each circuit, thus verifying itscondition, as well as the length of each circuit, enabling a schematicdiagram to be drawn in an automated fashion by the PDA 1, and printed ona printer 22.

The PCA 2, as disclosed in detail below, contains an embeddedmicroprocessor, dedicated analog measuring circuitry, and a radio linkfor wireless bi-directional communications with the PDA via a shortantenna 11. A separate test cord 12 connects the PCA 2 to one of severalSafeWire™ Sensor Probes 14 through connector 13. A simple probe, forexample, would test the ground resistance of switch plates, appliancesand the like, to determine whether they are correctly grounded or not.Another such probe is a Novel SafeWire™ Magnetic Field Probe, asdisclosed in detail in Section 7, that employs a novel method based onthe SafeWire™ system to better locate and trace wires in a wall or inconduit. Yet another is a Non-contact Infrared Temperature Sensor Probe,which can be used to sense the temperature of lamp fixtures. Otherspecialized sensing probes can be made available as needed.

The third basic component of the SafeWire™ system is the Load CenterInterface or LCI 15, which attaches to the home's Load Center 20 andserves to monitor signals in the Load Center that result from activeload testing performed by the PCA 2, for example, as the electricianmoves from outlet to outlet, fixture to fixture, appliance to appliance,and so on, throughout the house. The LCI 15 also contains an embeddedmicroprocessor, dedicated analog measuring circuitry and a Bluetoothradio link to enable wireless bi-directional communications with the PDA1 via its own short antenna 16.

A standard single-conductor coaxial “umbilical” cable 17 connects theLCI 15 to the PCA 2. This umbilical cable 17 may be coiled up inside thePCA by means of a spring-loaded reel mechanism that extends and retractsthe cord as the electrician moves about. The length of the umbilicalcable 17 should be on the order of 100 feet long to reach from the LoadCenter 20 to any outlet in a typical home. One or more umbilicalextensions can be used as needed for unusually large homes.

It is envisioned that the PCA 2 will be carried from room to room in thehouse to be examined by the electrician using handle 21, then placed onthe floor or a convenient surface near the outlet or lamp socket to betested. The test cable 3 of the preferred embodiment, shown separatelyin FIG. 3 and having attached to it the cable 5 with the appropriateadapter 6, is then plugged into the outlet to be tested. With a testcable 3 say 25 feet long, the PCA 2 can remain stationary as theelectrician moves from outlet to outlet in a room, testing each insuccession (the testing process itself being described in detail below).The PCA enclosure is preferably weighted enough to remain stationaryagainst the slight tugging that may result and may include storagespaces for the PDA and the various adapter cables.

The umbilical cable 17, though presenting some inconvenience to theelectrician, serves multiple useful purposes and as such is an importantcomponent of the present invention. First, provision of the umbilicalcable 17 allows hard-wired, secure electrical connection of the PCA 2 tothe Load Center 20. Among other advantages, this fact provides a knownground reference for use by the PCA to test branch circuits, which isparticularly useful when testing older ungrounded 2-prong outlet types,and in the determination of whether a particular outlet is completelydisconnected from the system or not. Second, the umbilical cable 17carries a DC voltage to the PCA to continuously power the microprocessorand analog circuits within, so that a separate power supply, with theattendant batteries, chargers, and the like, is not required. Third,umbilical cable 17 serves as a wideband conduit for high-frequencysignals used by the PCA to measure the length of wires from the outletunder test to the Load Center. Fourth, umbilical cable 17 is used tocarry high-speed communication signals between the LCI and the PCA. Morespecifically, while in the preferred embodiment of the SafeWire™ systemthe PDA 1 communicates with the PCA 2 via a Bluetooth radio link, theelectrician and his PDA usually being in the same room as the PCA, it isa distinct advantage to have this hardwired communication link betweenthe PCA 2and the LCI 15, because often the Load Center 20 will be somedistance away (perhaps out of Bluetooth radio range) and in a basementor other location where radio communications may not work well. Thus,the Bluetooth radio link indicated by antenna 16 on the LCI 15 of FIG. 1is only used when direct radio communication between the PDA 1 and theLCI 15 is required, as for example, when setting up the LCI 15 beforethe PCA is connected, or when the PCA 2 is out of radio range. A fifthuseful application of the umbilical cable 17 is as a conduit to carryload current back to the Load Center for the purpose of locating andtracing hidden wiring in accordance with the novel SafeWire MagneticField Probe of the present invention. It is apparent, therefore, thatthe use of the umbilical cable 17 enables these many features of theSafeWire™ system that would otherwise not be possible.

The fourth and final basic component of the SafeWire™ System is theMicro-Energy Dielectric Tester (μEDT) 18, an optional component used toperform non-destructive voltage breakdown tests on the wiring, bytesting the dielectric qualities of the insulation. Such dielectrictesting is useful, as it can reveal developing parallel faults that mayresult in arcing. However, it can only be performed on branch circuitswhere no loads are connected (so that there is no connection between thecircuit conductors); for example, only when all loads have beenunplugged or all light bulbs have been unscrewed a couple of turns. Ifsuch testing is desired, it is most conveniently done using thefollowing procedure: as the electrician moves through the house duringtesting using the PCA 2 she can simply unplug and leave unplugged (orswitched off or unscrewed) every load in the house. She then goes to theLoad Center 20, turns all power off, removes the LCI 15, attaches theμEDT 18 and proceeds with dielectric testing, a process that takes onlya few seconds for each branch circuit. If a load is inadvertently leftplugged in on one branch, the μEDT 18 will not damage it; it will simplyreport to the electrician that there is a load present, instructing herto either remove it or skip testing that branch circuit.

There are significant advantages to testing homes in this order. As willbe seen, PCA testing will show not only the resistance of each wire andconnection but also the length of each wire and what it is connected to.After simply and quickly moving through the house and connecting the PCAto each load, fixture, outlet, and appliance, while recordingidentification data as to each, the PDA will have collected enoughinformation to produce a complete schematic of the house. Now if duringsubsequent μED tests a potential parallel arc fault is found, e.g., thewires are somewhere too close together, the μEDT will reveal the faultand report the length of wire from the Load Center to the fault in feet.Since PCA testing has already measured and compiled the wire lengths toeach and every outlet and light socket in a home, the system has thedata to automatically translate this distance, i.e., the length of wirefrom the Load Center to the fault, into the actual location of thefault. For example, a fault that is 33.2 feet from the Load Center onBranch Circuit 3L might be better reported as a fault between outlets 2and 3 in the master bedroom, 4 feet to the left of outlet 3.

An alternate embodiment of the system is shown in FIG. 2, the onlydifference being that the signal processing circuits of the PCA 2 arecontained in a separate enclosure 23 that is located near the adapterplug 4 (one possible physical embodiment of this PCA enclosure isillustrated as 23 in FIG. 4), while the reel mechanism 25 for convenientstorage of the umbilical cable 17 is housed separately. The reason thisphysical implementation may be preferred over the embodiment of FIG. 1is because the signal processing circuits PCA are located closer toadapter 4, which may improve circuit performance. For example, theSafeWire™ distance measuring technique is based on sensing the time ofarrival of a pulse transmitted down the household wiring from the LoadCenter, and the length of test cable 3 in FIG. 1 may degrade the signal.Another example is that during load testing, the resistance of the testcable 3 will be part of the measured resistance. It is anticipated bythe inventor, however, that signal conduction problems with a longertest cable 3 can be resolved satisfactorily using various compensationtechniques. Note that although the Multi-function sensor probe 14 is notshown in the embodiment of FIG. 2, it is nonetheless part of thisembodiment of the invention.

The preferred embodiment of FIG. 1 also has advantages. As noted, thepurpose of standard SafeWire™ testing is to test every outlet in a home.Some of these outlets will not be easy to access—an outlet behind aheavy dresser, for example. Because there may barely be enough room toreach the outlet, the test cable of the preferred embodiment shown inFIG. 3, wherein the adapter cable assembly is no bigger than a commonextension cord, is clearly advantageous over that of FIG. 4, whereinprovision of the circuits of the PCA are housed in a device adding bulk.Another more subtle reason for the preferred embodiment is that the loadresistors in the PCA dissipate a lot of energy during continuous testingand therefore get quite hot, and the larger enclosure of the combinedPCA/reel of FIG. 1 can accommodate a larger and therefore more effectiveheat sink. One final argument for the preferred embodiment of FIG. 1 isthat the larger toolbox-styled PCA can be used to store adapter cablesand the PCA during transport.

The physical design of the LCI 15 is also important to the ease of useand therefore the economic viability of SafeWire™ testing. The LCI 15must be capable of being readily attached to a wide variety of LoadCenters quickly and safely in order for the electrician to be able totest a typical home in a couple of hours. To this end, the LCI 15 isdesigned in a modular fashion that is adapted to temporarily attach to aLoad Center and makes use of novel SafeWire Magnetic Probes, disclosedin detail elsewhere within this application, to make the temporaryelectrical contacts needed. These magnetic probes serve to convenientlyattach sensing wires to each branch circuit by magnetic attachment tothe steel head of the circuit breaker wire clamp bolts.

The preferred physical embodiment of the SafeWire™ Load Center Interface(LCI) 15 is shown in use, that is, temporarily attached to a typicalhousehold Load Center 20 in the illustration of FIG. 5. The Load Center20 itself comprises an enclosure 27, a main circuit breaker 28, and avariety of branch circuit breakers including standard single-pole 29,double-pole 30 and half-wide 31 types. A Ground bus 32 and a Neutral bus33 are also shown, the Neutral bus having an optional grounding strap 34to connect Neutral to Ground. For clarity, the Load Center wiring itselfis not shown. This type of Load Center is typical and the wiring andfunctionality of its components are well-known to electricians.

In the preferred embodiment illustrated, the LCI 15 comprises a MainModule 35, which clips on the bottom lip of the Load Center enclosure,and one or more Input Modules 36, 38 which stick to the metal flanges ofthe enclosure 27 via magnetic strips embedded in the back of the InputModules. One Input Module 36 serves the left side and is plugged intothe Main Module 35 via cable 37. A second Input Module 38 serves theright side and is plugged into the Main Module 35 via cable 39. EachInput Module accepts up to 16 Magnetic Probes, one for each circuitbreaker pole. If the Load Center has more breaker poles than this,additional Input Modules can be stacked on each side by simply pluggingthem in to existing ones.

For each branch circuit in the Load Center, a probe wire 41 is firstplugged into an input on an Input Module and then a Magnetic Probe 40 istemporarily attached to the wire clamp bolt 42 of the respective branchcircuit breaker. The magnet in the probe is encased in an extendedmolded polymer probe which keeps the electrician's fingers away from thelive terminals and makes it possible to attach the LCI to a “live” LoadCenter.

Power to the LCI and service feed sensing is obtained by attachingmagnetic probes to the incoming service feed, conveniently accessed atthe feed clamp-down hex bolts 45 in the main breaker 28. The magneticprobe 46 is shown spaced away from the hex bolt 45 for clarity. Thediameter of this probe 46 is a bit larger than the other probes 40, tobetter fit the larger bolts and to prevent mistakenly attaching them tobranch circuits, and is labeled accordingly. Although power could be fedinstead into the main module 35, in this case it is conveniently fedinto the expansion bus connector 47 on one of the Input modules.

The Main Module 35 contains a microprocessor that controls the operationof the LCI, connecting to each Input Module via the LCI bus and cables37 and 39, and to the PCA through the umbilical cable 17. In the drawingof FIG. 5, the Main Module 35 includes voltage indicators 210 and 211for each phase and a communications activity indicator 209. Theumbilical cable 17 is provided with a strain-relieving support (shownsimplified here, at 44) physically supporting it on the Load Centerenclosure, and plugs into the Main Module 35. The strain relief 44 isneeded to prevent disturbing the LCI 15 while the umbilical cable 17 istaken through the house.

The LCI with Input Modules connected, as shown in FIG. 5, has theability to identify which branch circuit the PCA is plugged into, thisinformation being used to document the system and produce a Load Centerlabel or schematic diagram. In some cases this information is not neededand the Main Module 35 of the LCI can be used alone with the power cable47 plugged directly into the LCI. SafeWire™ PCA testing can proceed asbefore, the only difference being that no information is gathered aboutwhich branch circuit the PCA is connected to.

Dielectric Testing using the Micro-Energy Dielectric Tester (μEDT) 18

The SafeWire™ System Micro-Energy Dielectric Tester (μEDT) 18 is shownattached to a typical Load Center enclosure 27 in FIG. 6. The method bywhich this testing is done is described in U.S. Pat. No. 6,777,953issued to the present inventor. A low current, high compliance currentsource is connected across the line to be tested to charge the interwirecapacitance to a high voltage (3,000V) over a short period of time (<1sec). If any loads are inadvertently present, the low current source (5μA) will not charge the wires to more than a couple of volts, the testwill not be run, and the “Load Present” lamp will be lit, alerting theelectrician to the situation. If no loads are present, i.e. nothing atall is plugged in, the voltage will ramp upward toward the full 3,000volts. If a fault is present it will spark over once, with an energylevel similar to the static discharge one may experience when walkingacross a carpet and then touching a metal doorknob. From this singledischarge, the μEDT 18 can determine the distance to the fault, bymeasuring precisely the time between arrivals of the initial pulse andthe first reflected pulse, as described in U.S. Pat. No. 6,777,953. Thistechnique can be applied to a home Load Center by first turning off theMain Circuit Breaker and all Branch Circuit Breakers. The μEDT currentsource is then connected to one phase of the Load Center Bus, which atthis point is not connected to anything. Then each Branch CircuitBreaker is turned ON in succession to test each individual circuit. Thismethod places the μEDT 18 at the end of a transmission line formed bythe cable of the branch circuit, a requirement of the method in order toproduce valid reflected pulses from which the distance to the arc can bedetermined.

Again referring to FIG. 6, the μEDT 18 is clipped on to the bottom lipof the Load Center by means of an integral clip on the back of theenclosure. Power for the tester is derived from power cable 49 that usesmagnetic probes 52 to attach to L1 and L2 on the input screw terminalsof the Main Breaker 28 and Ground on the Ground bus 32 (L1 and L2 referto Line 1 and Line 2, typically labeled Phase A and Phase B). In thismanner, the μEDT 18 is powered even when the Main Breaker 28 is OFF.Test cables 50 and 51, which carry the outputs of the μEDT (one for eachphase), must be connected to the L1 and L2 Load Center buses. This isconveniently accomplished by attached the leads 50 and 51, usingmagnetic probes, to two spare breaker poles, one on each phase. If adouble-pole, 220-volt slot is available, then a two-pole breaker 54 cansimply be plugged in and the two Dielectric Test cables 50 and 51attached as shown. If two single-pole breaker positions are available,one on each phase, then the probes 50 and 51 can be connected to them.If no open positions are available, then it will be necessary totemporarily disconnect the load on one double pole breaker, the electricrange for example, and then attach the magnetic probes 50 and 51 to thatbreaker.

As discussed previously, if dielectric testing is to be done, every loadwill have been left unplugged and all light circuits to be tested willhave had each bulb unscrewed a couple of turns and the light switch leftON so as to enable dielectric testing of both the light socket and thewiring between the light switch and the light socket. This is mostconveniently done during PCA testing. The specific test procedure is tofirst turn all circuit breakers OFF including the MAIN breaker, followedby attachment of the μEDT 18 to the Load Center as shown in FIG. 6. ThePDA 1, which communicates with the μEDT 18 through a wireless radio linkindicated by antenna 53, will then instruct the electrician to turn eachbranch circuit breaker ON in sequence. Within a couple of seconds aftereach breaker is turned ON, the test will be automatically completed, thePDA will instruct the electrician to turn it OFF and then turn ON thenext breaker. If a load happens to still be present on the branchcircuit switched ON, the PDA will so indicate and not test that circuit.If desired, the load can be found, unplugged and the test repeated. TheMicro-Energy Dielectric Tester 18 is completely safe and non-destructiveto any loads that might be on the circuit.

If a parallel fault is present the PDA 1 will display which two wiresthe fault is between and the distance from the Load Center to the fault.Since the PDA 1 already has a complete map of the household wiring, itcan then indicate to the electrician precisely where the fault is, e.g.,at outlet #3 in the Master Bedroom.

SafeWire™ Measurements

Wire Resistance—The system measures the source resistance of each wireat the adapter plug using the novel AC Balance Method of the presentinvention. Basically the PCA applies a rectified resistive load betweeneither Hot and Neutral, or between Hot and Ground, causing about 10amperes to flow and producing a voltage drop on each wire responsive tothe current flowing though it. These individual voltage drops aredetermined by measuring the average DC value of the line voltage at theadapter cable. From these voltage drop measurements, the total sourceresistance of each wire can be calculated.

Load Center Source Impedance—The wire resistance measured above includesthe impedance of the distribution transformer and the service feed lineto the house. By measuring the shift in DC average of the line voltageinput to the Load Center as the PCA applies the rectified load at theadapter, the source resistance of the Load Center can be determined. Bysubtracting this source resistance from the measured resistances at theadapter plug, the resistance of the wires from the plug to the LoadCenter (including the resistances of the Main Circuit Breaker and theBranch Breaker) can then be determined.

Circuit Breaker Resistances—The PCA applies the rectified load for about1 second, which is 60 line cycles at 60 hz. There are two reasons forapplying the load for so long. First, degraded contacts often exhibit ahigher resistance only when heated so it is necessary to apply the loadfor a long enough time for the temperature to rise at the contactjunction. Second, when the load is applied, the LCI measures the voltagedrop across each Branch Circuit Breaker for the purpose of determiningwhich Branch Circuit the load is on. The LCI measures the current over asingle cycle and samples each breaker in sequence. Thus if the load isapplied for 60 cycles, up to 60 breakers can be sampled.

Since the applied load current is substantially constant (fixedresistor), the voltage measured across the breaker will be proportionalto the resistance of the breaker. In the preferred embodiment, the LCIsamples at the input to the Main Breaker and at the output of eachBranch Breaker, i.e., across the two breakers. Thus the measuredresistance is the sum of the two breaker resistances. The small seriesresistance of a breaker is due to the bi-metallic trip element and isinversely proportional to the current rating and consequently theresistance of the Main Breaker (typically 200 amps) is much smaller thanthe branch breakers (typically 15 or 20 amps).

Finally, with the breaker resistance thus measured during a PCA loadtest, this resistance can also be subtracted from the measuredresistance to determine the wire resistance from the adapter to the LoadCenter, and any abnormal or dangerous conditions brought to theattention of the electrician via the PDA 1.

Branch Circuit Number—As discussed above, the LCI scans all the BranchCircuit Breakers during a PCA load test and can therefore correlate theBranch Circuit into which the PCA adapter is plugged to thecorresponding branch circuit. This information is then used by the PDA 1to produce a list cross-referencing the branch circuits with the variousappliance, rooms, fixtures and the like throughout the house, for futurereference and troubleshooting.

Wire Lengths—The PCA measures the length of each wire from the adapterto the Load Center in the following manner. It sends a fast step pulsedown the umbilical cable 17 to the LCI, which couples the pulse to theLoad Center, sending it out in all directions. The PCA, knowing thefixed delay due to the length of the umbilical cable 17, starts to lookfor a pulse edge to arrive at the adapter at the end of this delay. Onreception of the pulse edge, the PCA measures the time delay from whenit initially sent the pulse down the umbilical cable. Since the “Romex”cable typically used for residential wiring acts as a transmission linewith a consistent propagation velocity, the length of the wire can bedetermined from the measured delay.

Wire Gauges—With the resistance of the wire and its length known, thePDA can calculate and report the gauge of each individual wire. Whenthere is excessive resistance on one wire due to a bad connection, forexample, the system can sort it out by taking the lowest of the threewire resistances to estimate the gauge. More specifically, if the threewires on a grounded outlet are presumed to be the same length, and oneof the three wires exhibits a resistance under load that is higher thanthe other two, the system can surmise that a connection on that wire isthe cause of the increase in resistance, and therefore use the lowestresistance wire to estimate the gauge.

Arcing—The LCI employs a swept radio-frequency arc noise detectorconnected to the Load Center that monitors the entire house for thepresence of arcing. U.S. Pat. Nos. 5,729,145, 5,223,795, 5,434,509, and5,223,795, all issued to the present inventor, relate to the detectionof arcing by monitoring high-frequency emissions. One of therequirements of involved in differentiating arcing from other conditionsis the differentiation of RF noise, from radios, lamp dimmers, and thelike. Since the electrician has control of the house during SafeWire™testing, most loads will be turned off, keeping background RF noise to aminimum.

During PCA load testing the LCI monitors for the presence of arcingwhile the load is applied. In this case, extraneous impulse noise fromlamp dimmers and the like is further minimized by the principal oftime-interval filtering, i.e., sampling for arcing only during the shortinterval when the load is applied.

The normal arc that occurs when a light switch is turned ON or OFF isalso monitored by the LCI. The duration of the arc is displayed on thePDA and the electrician is notified if it is too long, indicating thatthe switch is old and should be replaced.

Appliances and other electrical devices can also be tested for excessivearcing. The electrician simply plugs the device in, sets the PDA tomonitor arcing and then turns the device ON and OFF, and perhaps run thedevices through its various operational modes.

Dielectric Breakdown Voltage—As discussed above, the μEDT 18 measuresthe breakdown voltage between Hot and Neutral or Ground on unloadedBranch Circuits and, if a breakdown occurs, reports the wire length fromthe Load Center to the fault. Since the PDA has a complete map of thehousehold wiring, it can then locate the fault precisely.

Correct Outlet and Light Fixture Wiring—The PCA has circuits to analyzethe wiring of outlets and light sockets and can report directly if theyare wired incorrectly, and can also tell the electrician precisely howto fix it.

Ground Fault Circuit Interrupters (GFCI)—The PCA can switch a 5milliampere load resistor (or any other value) from Hot to ground totest the functionality of installed GFCI's.

Arcing Fault Circuit Interrupters (AFCI)—The PCA can be programmed toswitch test loads in and out in a chaotic pattern to test some AFCI's.

Extension Cord Testing—An extension cord can be tested for arcing,individual wire resistances and breakdown voltage by plugging it into apreviously tested outlet and informing the PDA of the presence of thecord. This facility is particularly relevant to extension cordsinstalled in a semi-permanent manner, which may be undersized and canbecome dangerously degraded over time.

Appliance and other Metal Surface Grounding—A simple probe would testthe ground resistance of switch plates, appliances and the like, todetermine whether they are correctly grounded or not.

As described above, the PCA ground test probe can be used to quicklytest painted or unpainted metal surfaces to determine if they aregrounded. The probe also determines whether the surface has a properseparate ground path home.

SafeWire™ Documentation

SafeWire™ testing enables an individual electrician to test everyelectrical wire, every connection, every outlet, every switch, everylight, and every appliance in a house, typically in a few hours or less.Each test provides the length and resistance of each wire from thereceptacle to the Load Center, the voltage on each wire, the branchcircuit number, etc., the data typically including hundreds ofmeasurements or more. Most of this data need not even be seen by theelectrician because the SafeWire™ system automatically qualifies thedata pointing out the electrician only abnormal measurements oranomalies that require her attention. This complete set of data,however, is useful for a number of purposes, and is thereforeautomatically compiled and saved by the system in what the presentinventor refers to as the SafeWire™ Data Tables. These Data Tables arepreferably saved in a standard spreadsheet format, such as MicrosoftExcel, so that they may be conveniently accessed by either theelectrician or the homeowner. Furthermore, the SafeWire™ Data Tables canbe conveniently saved onto a small memory card, preferably one that canbe written to and read from by the PDA such as the commonly-available SDmemory card or its equivalent, and the card left in a small paper pocketon the inside of the Load Center door, for example.

In accordance with the present invention, the information saved inSafeWire™ Data Tables can be used to generate a Load Center Label thatcan be printed directly onto special magnetic “paper” and thenconveniently stuck to the metal door of the Load Center. Magnetic“paper” that can be printed on in a standard inkjet printer is readilyavailable from a number of commercial sources. The Load Center Label canbe made to list the number and types of lights, outlets, and even thenames of specific loads, e.g., refrigerator, on each branch circuitbreaker. The information in SafeWire™ Data Tables can be further used togenerate a schematic diagram of the house including the lengths of eachwire. To do this a computer, either the PDA or a separate desktopcomputer, processes the raw data which including all measured wirelengths, to generate a complete schematic diagram of the installation,noting or resolving any ambiguities that arise. Finally, the SafeWire™Data Tables can be used to advantage to run Aging Wiring ComparisonReports, whereby an electrician can quickly and conveniently compare theresults of current SafeWire™ testing to that of previous SafeWire™testing recorded on the memory card, the report automatically pointingout the differences. This may be of particular value in testing museums,hospitals and other high-value electrical wiring installations.

2. The SafeWire™ Portable Circuit Analyzer (PCA)

The SafeWire™ Portable Circuit Analyzer or PCA 2 is a portable unit witha handle on top designed to be carried by an electrician throughout ahome to test outlets, light fixtures, appliance grounds, and otherelements of the electrical system of the house. In the preferredembodiment of FIG. 1, as discussed above, the PCA carries the umbilicalcable 17, a thin coaxial cable stored on a retractable reel thatconnects the PCA 2 to the Load Center 15 of the home. As the electricianwalks from room to room, he unreels the cable 17 from the PCA, placesthe PCA in a convenient location (typically on the floor), and thenconnects, for example, a short cable with the appropriate adapter toplug into the outlets to be tested. The electrician controls the testsand views the results on a specially-programmed handheld PDA 1, whichcommunicates with the PCA 2 via a built-in Bluetooth radio link.

A simplified schematic diagram of the PCA 2 is shown in FIG. 7. Anadapter connector 55 is disposed to accept a variety of adapter cablesthat plug into specific outlet types. One such adapter cable 56 adaptsthe PCA to a standard grounded 3-prong outlet 57. Identificationresistor 58 is used by the system to identify the type of adapterplugged in. A second adapter cable 59 is configured to connect to anolder 2-prong outlet 60 and also includes an identification resistor 61.Note that in this case no connection is made to the Ground terminal onthe adapter. A third adapter cable example 62 connects to a standard4-prong 220V appliance outlet 63 and again contains an integralidentification resistor 64. Many more types of adapters can be used tointerface to other types of outlets or other points at which circuittesting is appropriate, such as lamp sockets; in that case, aspring-loaded device adapted to be inserted into the lamp socket (i.e.,without requiring the electrician to thread a connector into the socket)might usefully be provided.

The PCA employs a microprocessor 76 to control load switching and takevarious measurements. The PCA communicates with the handheld PDA via RFlink 92 that feeds antenna 11 (antenna 24 in FIG. 8). Control lines 94control the exchange. A wireless link is advantageous because a numberof the desired suite of SafeWire™ tests are stimulus-response tests,wherein the electrician monitors the response (with the PDA 1 held inone hand) while initiating the stimulus with the other, for example, hemay switch a light switch on and off while observing the duration of thearcs produced. Or he may physically tap a lighting fixture whilemonitoring the display on the PDA for the presence of arcing. Since anumber of commercially available PDAs contain an integral Bluetoothradio, such as the Palm Pilot Tungsten series, inclusion of a radio datatransceiver in the PCA allows the electrician to conveniently view testresults on the Palm Pilot in one hand, while simultaneously executingtasks with the other, such as switching light switches ON and OFF.

An umbilical cable 17 is used to connect the PCA 2 to the LCI 15 at theLoad Center of the home. Conveniently, the umbilical cable 17 is storedin a retractable reel comprised by the PCA 2. The umbilical cable 17comprises a coaxial cable including a single conductor 89 and an overallshield as indicated at 88. The shield 88 at the other end of theumbilical cable connects to the circuit ground 91 of the LCI which, whenattached to the Load Center, is connected to the household Ground. Theumbilical cable 17 serves a number of purposes simultaneously.Low-voltage DC power from the LCI 15 is transmitted down the umbilicalcable 17 to power the PCA 2. Although power for the PCA could beobtained from the outlets that it is plugged into, this would mean thatthe PCA is only powered when plugged in. It is advantageous for the PCAto be constantly powered, so that communications with the PDA cancontinue, for example while the outlets are depowered, and thereforepower is supplied down the umbilical cable 17. The second functionserved by the umbilical cable 17 is that the shield 88 comprised therebyconnects the PCA circuit common 90 to the Load Center ground 91, thusproviding a voltage reference for outlet measurements. A third use ofthe umbilical cable 17 is that it provides a bi-directionalcommunications link between the PCA 2 and the LCI 17. While this toocould have been provided by a radio link, there are substantialadvantages to using a hard-wired link. The electrician and the PDA willalways be near the PCA, typically within 10 or 15 feet, and so aBluetooth link with a minimal range of 30 feet is acceptable for thiscommunication facility. But the Load Center is commonly much fartheraway, typically in the basement and in the case of apartment buildings,for example, perhaps several floors and one or more cement walls away.It is therefore advantageous to have a secure, hardwired link betweenLCI 15 and PCA 2. A further use of the umbilical cable 17 is as ahigh-speed conduit for the digital pulse used to measure the length ofwires in the house. For this reason, the umbilical cable 17 is coaxialcable with well-defined impedance and loss characteristics. Theinterface and switching circuits to multiplex these various functions,labeled UI for Umbilical Interface, are shown as 85 in FIG. 7. Themicroprocessor 76 controls the circuits through control lines 87.

The microprocessor 76 measures voltages on the four adapter lines 65using four Multi-Function Interface modules 69, which are configurabledevices controllable by the microprocessor 76 via control lines 68 toperform different basic functions depending on what measurement isdesired to be taken. Each MFI 69, for example, can be configured toserve as a resistive divider, a resistance measurement circuit or aLow-Pass Filter (LPF). It is a relatively simple matter for one skilledin the art to adapt the Multi-Function Interfaces to switchably performthese functions and therefore the circuits are not described in detailhere. Four current-limiting resistors 67 serve to sample the current onthe corresponding line; each resistor 67 is of a high enough value tolimit the current to milliamp levels that can be easily clamped andthereby allow low-voltage switching means to be used to change circuitconfigurations.

In a typical test sequence, the first task of the PCA is to configuretwo of the four Multi-Function Interfaces 69 to measure the value of theidentification resistor (e.g., 58, 61 or 64) before the adapter plug(e.g., 57, 60 or 63) is plugged into an outlet. If infinite, no adaptercable is plugged in. All other values correspond to a particular adaptertype allowing automatic identification by the PCA.

The second basic task is to verify that the wiring of the outlet orsocket is correct. The PCA configures the MFIs to form simple voltagedividers with resistors 67 to measure the voltages present on each ofthe four lines 65. Using this data the PCA can detect when the adapterhas been plugged in, and whether the socket under test is correctlywired. If a wiring error is detected, the electrician is informed of itdirectly on the PDA and given instructions on how to correct it.

If the PCA is to be used to test a GFCI (Ground Fault CircuitInterrupter) protected outlet, the electrician so indicates on the PDA,so that the PCA is advised, in effect, that a different test is requiredthan testing of an ordinary outlet, although the same adapter cable 5will be used. The PCA can test the GFCI by applying a resistor R_(G)between either L1 or L2 and ground, the test leakage current being setby the value of R_(G). Switch 129 applies test resistor 131 between L1and ground and switch 128 applies test resistor 130 between L1 andground. Since the ground used is the PCA circuit ground, which isconnected to the Load Center ground via the umbilical cable 17,ungrounded (2-wire) GFCI protected outlets can be tested. Finally, bymonitoring the voltage on L1 or L2 (through resistors 67) as R_(G) 130is applied, the GFCI trip time can also be measured.

The third test to be performed by the PCA is to measure the length ofthe wires from the Load Center to the outlet under test. Detaileddiscussion is provided in Section 6 below entitled “Least-TimePropagation Method”. The process is generally as follows. First themicroprocessor 76 applies a pulse with a fast leading edge to umbilicalcable 17 via a fast line driver in the umbilical interface 85, selectedby control lines 87. The output impedance of the driver should match thecharacteristic impedance of the umbilical cable 17. The leading edge ofthis pulse will travel down the fixed length umbilical cable 17 to theLCI, where it is coupled to the line between L1 or L2, as required, andG and N which are tied together there. The PCA, knowing the length ofthe umbilical and hence the delay before the pulse can arrive, waitsthat amount of time and then starts a constant input integrator 73, soas to begin timing the transit time and enables a programmable thresholddetector (PTD) 71 to begin looking for the pulse. When the leading edgeof the pulse reaches a high-frequency coupling unit 66 connected to eachof the wires 65 connected to the conductors of the socket under test, itis coupled to the PTD 71 which trips and stops the integration, holdingthe value until the microprocessor 76 is able to read it. If no pulse isdetected within a preset period of time, equal to the time required forthe maximum expected wire length, the PTD 71 is disabled and an errornoted. The threshold of the PTD 71 is desirably set to the lowestthreshold that does not cause excessive false tripping. On success, thesame measurement can be repeated several times to be sure it isrepeatable and therefore indicates a reliable result.

The fourth basic function performed by the PCA 2 is that of load testingeach of the four lines 65 to measure the source resistance of each lineusing the present invention's AC Balance method. This process isdetailed in Section 5 below entitled “AC Balance Method”. The followingsummary is provided here to allow more complete understanding of the PCA2. First, the MFIs 69 are configured to form four low-pass filters. Eachof these filters should be a 3-pole Bessel filter with a cutofffrequency of about 0.16 hz. Switches 81 and 82 allow the microprocessor76 to insert load resistor 83 between L1 and N or between L1 and Grespectively. Rectifier 84 allows current to flow through load resistor83 only during positive half-cycles of L1. Similarly, switches 79 and 80allow the microprocessor 76 to insert load resistor 78 between L2 and Nor between L2 and G respectively, and rectifier 77 restricts the currentto positive half-cycles of L2. As described in Section 5 below, eachswitch is closed in sequence to allow rectified current from either L1or L2 to flow to either Ground or Neutral or both. The average DCvoltage at the output of each low-pass filter is measured and from thesereadings the resistance of each line is calculated. The resistances fromthese measurements are the total source resistance feeding the outletand therefore can be used to determine the total current capacity ofeach outlet, assuming the NEC standard 3% maximum voltage drop. As thisdata is collected for each wire on each outlet, it is stored in tablesin the PDA for later compilation and reporting.

As noted, the PCA 2 operates by applying a load across differentcombinations of lines 65 and measuring the voltage drop produced alongthose lines using the AC Balance method. Both the size of the load andthe duration over which it is applied are important considerations. Ascontacts deteriorate, the current is gradually restricted to very smallcross-sectional areas, which heat up in proportion to the square of thecurrent flowing. For this reason, to expose such connections the testload current must be relatively high, on the order of 10 amperes fortypical household wiring. The duration of time that the load is appliedis also important because it takes time for these restricted bridges toheat up. Testing by the present inventor has shown that the load shouldbe applied for about one second to reliably reveal deterioratingconnections.

If other loads happen to be present on the line, there may exist avoltage drop on any one of the lines before any of the switches 79–82are actuated. Also, since the typical signal levels to be measured aremillivolts, circuit component offsets may also contribute to theunloaded offset. For both reasons, an improvement in the AC Balancemeasurement procedure is to first measure and record the average DCvoltages on each of the four lines. After the loads are switched on andvoltage measurements taken, these initial offset voltages are subtractedfrom the readings, thereby correcting for initial offsets.

It will be noted that while the purpose of the PCA 2 is to measure theresistance of the wires feeding the outlet under test, the additionalresistance of the four wires in the test cable 5 that extends from theoutlet to the point at which the sense resistors 67 are tied, and thecontact resistance of mating test connectors 55, 56 of that cableassembly, will also be measured. Since the resistances of these wiresand the connector are known and constant (presuming use of a goodquality connector), the PCA can simply subtract these known resistancesfrom the measured resistance values to compensate.

An alternative and more accurate means to compensate the test leadresistance is to run both a current-carrying wire and a smaller gaugesense wire from the PCA all the way through the connector that plugsinto the outlet under test. This configuration is illustrated in theschematic of FIG. 8. As shown, each sense resistor 67 now feeds aseparate wire that runs all the way through adapter connectors 55 and 56to test connector 57. In this manner, the wire resistances of the testcable 65 and the contact resistances of the adapter connectors 55 and 56are accurately compensated for. This method, commonly known as the4-wire resistance measuring method, is well known and made use of incommercially available low-resistance measuring instruments.

One useful purpose of using the AC Balance method to measure wireresistances under load in the SafeWire™ system is to identify and locateundersized wires. Since the PCA is capable of measuring the actual wirelength and its resistance, it becomes a simple matter to calculate thegauge of the wire. The AC balance method disclosed so far, however,measures the total source resistance as seen at the outlet, whichincludes the wire resistance from the Load Center to the outlet, thebranch circuit breaker resistance, the Main Service Breaker resistance,the resistance of the Service Feed cable and the source resistance ofthe residential Distribution Transformer. There exists a need,therefore, to determine the actual resistance of the wires from theoutlet to the Load Center.

The SafeWire™ system makes use of two more novel methods to separatethese resistances. Because part of the circuitry employed to this end iscontained in the LCI module attached to the Load Center, it is nownecessary to make some reference to the SafeWire™ system as a whole.FIG. 9 shows a simplified schematic diagram of the AC Balance portion ofthe PCA 2 plugged into a 220-volt outlet 114 on a household wiringsystem. This could of course be any other type outlet or lamp socketinstead of the 220-volt outlet shown. A second outlet 115 on the samebranch circuit has nothing plugged into it. A Residential DistributionTransformer 105 feeds through a Service Entrance (not shown) to the MainBreaker 108 in the Load Center 107. The branch circuit shown isprotected by Branch Circuit Breaker 110. Other branch circuits (also notshown) feed off of lines 109 and 112 in the Load Center. In accordancewith accepted NEC standards, the Load Center is connected to an EarthGround Rod 111 and the Ground and Neutral busses are bonded together atpoint 113. The Distribution Transformer 105 typically has a separateEarth Ground Rod 106 located in the vicinity of transformer 105.

The Load Center Interface (LCI) 15, described in detail in Section 3below entitled “SafeWire™ Load Center Interface”, is electricallyattached to the Load Center at various points. An umbilical cable 17,between the PCA 2 and the LCI 15 serves to connect the circuit ground 90of the PCA to the circuit ground 91 of the LCI and to allow themicroprocessor 76 in the PCA 2 to communicate with the microprocessor121 in the LCI 15.

Three wires from the LCI 15 attach to the Load Center to both supplypower to the LCI (power supplies not shown) and to sense the incomingpower lines. Two of these attach to the input screw terminals of theMain Breaker to sense L1 and L2 through current-limiting resistors 117and 116 respectively, and a third connects the LCI circuit ground 123 tothe Neutral bus. All three connections are made safely and convenientlyusing novel magnetic probes as described in Section 4 below, entitled“SafeWire™ Magnetic Probe.” Microprocessor 121 in the LCI measuresvoltages on L1 and L2 using two Multi-Function Interface modules 118 and119, which as in the PCA are configurable devices controllable by themicroprocessor 121 via control lines 122 to perform different basicfunctions depending on what measurement is desired to be taken.

As will be recalled from the previous discussion of the AC BalanceMethod and as discussed further in Section 5 below entitled “AC BalanceMethod”, the PCA 2 applies a rectified load resistor across variouscombinations of lines and measures the voltage developed on those lines,from which data the resistance of the lines can be determined. Theresistance of each line is the total source resistance for that line,including the wire resistance from the outlet under test to the LoadCenter, the branch circuit breaker resistance, the Main Service Breakerresistance, the resistance of the Service Feed cable and the sourceresistance of the residential Distribution Transformer. By measuring theshift in DC average at the input to the Main Breaker (by configuring theMFIs 118 and 199 as low-pass filters in the same manner as in the PCAdescribed above), at the same time that the rectified load is applied atthe outlet 114, the LCI 15 can now determine the total source impedancefeeding the Load Center, including the impedance of the distributiontransformer 105 and the service feed line. Since the voltages aremeasured independently on L1 and L2, the source resistance of the twophases can be determined independently of each other. From this data,the LCI can readily calculate, based on the accepted NEC standard of a3% maximum voltage drop, the service capacity of the house in amperes(typically 100–200A). Because the impedance of the Service Feed will bean order of magnitude or so lower than the resistance of wires on anindividual branch circuit, it is advantageous to make the low-passfilter configuration of Multi-Function Interfaces 118 and 119 moresensitive to DC level shift than the ones incorporated in the PCA.

Thus the first novel method to separate the resistances is to measurethe Service Feed impedance to the house and subtract it from the totalmeasured impedance. For maximum accuracy, this can be done every time anoutlet or lamp circuit in the house is load tested. At this point, thecorrected resistance of a wire measured by the PCA still includes theseries resistance of the Branch Circuit Breaker and the Main CircuitBreaker. A separate feature of the LCI can be used to measure the seriesresistance of the Main Breaker and each Branch Circuit breakerindividually. With this last piece of data, the LCI can determine whatthe wire resistance is from the outlet under test to the Load Center,and using the measured length of the wire, calculate the wire gauge.

As described in detail in Section 3 below, entitled “SafeWire™ LoadCenter Interface”, the LCI measures the voltage drop across each of thebranch circuit breakers in order to identify the branch circuit to whichthe outlet under test is connected. All circuit breakers of the typeused in residential wiring have a thermal element that heats up andbends when more than a predetermined amount of current flows through it;a mechanical device then opens the circuit. The reason the internalbi-metal element heats up is that it has a resistance, albeit only onthe order of milliohms, but this is enough for the LCI to detect loadcurrents as low as an ampere or less. Thus one major function of theLCI, identifying which branch circuit the outlet under test is on, isaccomplished by looking for a voltage drop across one of the branchcircuit breakers when the rectified load is applied during load testing.Since the load current, being determined by the load resistance R_(L1)or R_(L2) is substantially constant, the voltage drop across the breakeris proportional to the series resistance of the breaker.

In fact, in accordance with FIG. 9, the resistance that will normally bemeasured in this fashion is the series resistance of the Main Breakerplus the series resistance of the Branch Breaker selected. The reasonfor this is simply that it is inconvenient to access the internal busesof the Load Center with the magnetic probes, which would be necessary inorder to make these connections “downstream” of the Main Breaker. Havingthe Main Breaker in the circuit makes little difference because theseries impedance of the Main Breaker is an order of magnitude less thanthe series resistance of a branch breaker.

The purpose for measuring the series resistance of the breakers istwofold. First, a common failure that sometimes leads to house fires isarcing at the press-on tab contact on the back of branch circuitbreakers. If a tab has been arcing it will exhibit a higher contactresistance under load and thus be revealed with this test (theradio-frequency noise of any arcing that occurs will also be detected bythe LCI, see Section 3 below). The second reason is that the contactsinside an aged circuit breaker may deteriorate to the point that theybegin to arc. This too would be readily revealed by a higher than normalseries resistance under load.

Returning to discussion of the circuit of the preferred embodiment ofthe PCA 2 shown in FIG. 7, a SafeWire™ Multi-Sensor Probe interface isprovided, including connector 95. This interface is included to supporta variety of small hand-held sensors that can be used to advantage bythe electrician while testing. A simple Ground Safety Probe, forexample, using conventional resistance measuring circuits can test theresistance between metal surfaces on switch plates, appliances and thelike, and Ground at the Load Center (using the umbilical cablereference). An infrared temperature-sensing probe could be used by theelectrician to sense spot temperatures at light fixtures. A contacttemperature-sensing probe might also be useful. It is often advantageouswith sensing probes of this type to place the low-level analog circuitsat the probe itself and, for this reason, a generic sensing probeinterface is provided.

In FIG. 7 a generic sensing probe is shown in simplified schematic form.The probe assembly, located at the end of a convenient length of cable,comprises a number of generic elements. The output of sensor 97 isconditioned and amplified by analog circuits 98 to feed the line markedS (Sense) on connector 96. Power to the probe is provided by theV(Voltage) and G(Ground) lines. Provisions are also included to allowthe PCA to automatically identify what type of sensor module is pluggedin. The line marked I (Identification) on connector 96 is fed by aresistive divider formed by resistor 100 and resistor 101. By assigningthe value of resistor 101 in accordance with the sensor type, thevoltage on line I can be read by the microprocessor 76 to identify theprobe. This same line can be used to determine whether any probe isplugged in (the voltage on line I will be at V+ when no probe is pluggedin) and to initiate a measurement by pressing normally open switch 102(the voltage on line I will be 0 when switch 102 is pressed). One ormore LED indicators 103 may be included to provide a Pass/Fail visualindication to the electrician at the probe.

The simple Ground Safety Probe discussed above can usefully employanother novel method of the present invention to determine whether aseparate ground return path is present as required by most currentelectrical codes. For well-known safety reasons, modern groundingstandards dictate that no current should normally flow through aprotective ground wire. The method comprises first measuring theresistance between the grounded surface and the Load Center Ground and,thus established, monitor the voltage on the grounded surface as theassociated load is turned ON. For example, if the load is a dishwasher,touch the probe to an exposed metal surface, measure the groundresistance and then momentarily turn the dishwasher ON. If this voltagerises responsive to load current, it reveals that the load current isflowing, at least in part, in the Ground wire. On a correctly groundeddishwasher, the load current will flow back through the Neutral wire andno voltage will be seen on the Ground.

Finally, the PCA may also be used to test extension cords, outlet stripsand the like. Once the PDA has determined the source impedance of anoutlet, as described earlier, an extension cord, for example, can beplugged into the outlet and the PCA then plugged into the end of theextension cord with the appropriate adapter. The electrician informs thePDA of the extension cord and the system then calculates the resistanceof each wire in the cord by simply subtracting the source resistance ofthe outlet from the total measured resistance on each wire.

3. Safe Wire™ Load Center Interface (LCI)

The SafeWire™ system Load Center Interface or LCI 15 attaches to theLoad Center in a home and serves to monitor various parameters as theelectrician moves throughout the home testing outlets, appliances,switches and light circuits with the PCA. The LCI is an expandablemodular unit designed to be easily, safely and quickly attached andremoved from a Load Center using a variety of magnetic attachment meansincluding a novel Magnetic Probe design (see Section 4).

A simplified drawing of the LCI 15 as attached to a typical home LoadCenter is illustrated in FIG. 5, and the physical aspects of theconnection scheme employed were discussed above.

One purpose of the LCI 15 in the SafeWire™ system is to determine whichBranch Circuit the PCA 2 is connected to as the electrician movesthroughout the house, testing outlets and light circuits. To accomplishthis the LCI 15 must be able to identify each breaker by position, andto measure the current individually flowing through each breaker (and,in order to support the PCA's function of providing a Load Center labeland a printed circuit diagram of the house, the LCI must able tocommunicate this information to the PCA via umbilical cable 17). Whileboth magnetic and Hall effect clamp-on AC current sensors are readilyavailable, they are generally too large and too expensive to bepractical for simultaneous connection to every breaker in a Load Center.If a custom clamp-on current sensor could be developed that isinexpensive, very small and made to clamp tightly on to 8–14 gaugewires, it could serve the current purpose well and is thus considered tobe within the scope of this application. The method disclosed herein,however, is much less expensive and much more practical than magneticcurrent sensing and therefore better suited to the need.

The novel method of the present invention is to sense current flowthrough the breaker by measuring the voltage drop across it. Theinventor, noting that all home breakers contain an integral thermal tripelement, which by its nature exhibits a small resistance (the resistanceof the bi-metal trip element is what causes the element to heat up inresponse to excessive current and therefore bend, tripping the breaker),realized that the small voltage produced across the breaker could beused to sense current flowing through it. Furthermore, since the PCAapplies a substantially constant-current load during testing, themeasured voltage across the breaker can be used to calculate theresistance of the breaker. Tests by the present inventor have shown thatcurrents as low as 1 ampere can be measured accurately using thismethod.

Electrical systems built before circuit breakers were commonly availableusually employed fuses of various types. The current sensing method justdescribed also works with fuses because fuses too are thermal deviceswhose current interruption mechanism is based on the heat generated bythe current flowing through the fuse element's resistance. Powerdistribution systems that employ fuses instead of circuit breakers, inpart or in total, such as found in old houses for example, represent avaluable potential market for SafeWire™ testing. It is therefore to beunderstood that in the context of this application the term “circuitbreaker” shall include fuses of all types.

Load Centers may be configured as Main Service Panels or Sub-Panels andcome in a variety of types and sizes, ranging from just a few branchcircuits to forty or more. Temporary connections need to be made toevery Branch Circuit Breaker terminal and to the incoming power lines.The modular design of the LCI of the preferred embodiment of FIG. 5makes it quick, safe and easy to interface to a wide variety of LoadCenters. It is to be understood that other packaging methods and modularinterconnect designs may also work and are within the scope of thisinvention.

Referring again to FIG. 5, attachment of the LCI 15 to the Load Center20 may proceed generally as follows.

First remove the cover of the Load Center 20 and set it aside. Then clipthe Main Module 35 on to the lower lip of the Load Center enclosure 27.Starting on the right side, stick (by means of an integral magneticstrip on the back of it) an Input Module 38 to the exposed metal flangeof the enclosure 27, and position it so that the top is near the topBranch Circuit Breaker. The Input Module 38 shown has 16 input jacks onthe front and can therefore be connected to up to 16 circuit breakers.If the Load Center has more than 16 breakers on the right side, thenanother Input Module (not shown) can be simply placed above or below thefirst module 38 and plugged into it to increase the total number ofinputs to 32. Each Input Module has a connector on the top and thebottom, and the internal control bus is arranged so that they can bedaisy-chained in this manner. A control cable 39 attaches the bottom ofthe Input Module 38 to the Main Module 35. Now repeat this procedure onthe left side of the Load Center.

Next attach sensing wires to each breaker. Starting on the top left,plug a SafeWire™ Magnetic Probe Sensor Wire (detailed below) into thetop input jack and then stick the Magnetic Probe to the head of thescrew terminal on the first branch circuit breaker. An unattachedmagnetic probe 40 on sensor wire 41, which connects to circuit breakerscrew 42, is shown for reference. Proceed down the left side to attacheach successive circuit breaker pole to its corresponding input. If abreaker pole position is empty, skip that input jack. Proceed in thesame manner down the right side of the panel.

Power to the LCI is provided by the Power Input Cable 47, which is shownin schematic form in FIG. 13. The connector 173 plugs into the end ofany Input Module to supply power to the entire system. The Magneticprobes for L1 (174) and L2 (175) are physically designed to be suitablefor attachment to the input lugs of the Main Breaker and are thereforesomewhat larger than the Branch Circuit Breaker probes. Similarly, theground probe 175 is adapted in size to be suitable for attachment to ascrew on the ground bus bar.

Thus, with all Branch Circuit Breakers connected, plug a Power InputCable 47 into the top of one of the Input Modules, in this case Module36, and first stick ground probe 93 (preferably color-coded green) tothe top of one of the screws on the Ground Bus Bar 32. Next, carefullystick the black L1 probe 46 to one input terminal 45 of Main Breaker 28.Since the terminal is live, the corresponding L1 light on the MainModule 35 will illuminate. Carefully stick the red L2 probe to the otherincoming terminal of Main Breaker 28, and light L2 will illuminate.Finally, plug the umbilical cable 17 into the Main Module. The LCIinstallation is now complete.

The system is designed so that the LCI 15 can be safely attached by anelectrician to the Load Center 20 while the Load Center is live, i.e.,while the Main Breaker 28 is ON. The SafeWire™ Magnetic Probes are fullyinsulated and long enough so that the electrician's fingers need notcome near the live terminal. The magnetic probe sensor wire shouldalways be plugged first into the Input Module and then stuck to the liveterminal on the breaker instead of the other way around. SafeWire™Magnetic Probes are also each internally fused at a very low currentlevel for added protection. If desired, for safety, the Main Breaker canbe turned OFF during installation.

Referring now to FIG. 10, the basic measurement method of the presentinvention is shown in simplified schematic form. As discussed above, thetypical Load Center 53 is fed by a Service Feed from a distributiontransformer 105, which is typically grounded to Earth via a groundingstake 106. Three power lines, L1, Neutral and L2 feed the Load Center 53through a Service Entrance with L1 and L2 connected directly to atwo-pole Main Breaker 108 and the Neutral line connected to a NeutralBus Bar in the Load Center. A Separate Ground Bus Bar in the Load Centeris connected to the Neutral Bus Bar at 113 and both are connected tolocal Ground Stake 111. The output of the Main Breaker 108 feeds busesthat allow connection of a number of Branch Circuit Breakers, some fromL1 (134) and some from L2 (135).

The LCI 15 generally comprises a voltmeter on each phase, capable ofmeasuring the small voltage that develops across each circuit breakerwhen current flows through it, and a switching network to connect eachvoltmeter across any selected Branch Circuit Breaker. The voltmeter 137on L1, for example, has one end connected to the incoming L1 (138) andcan be connected through switches 139 to any one of the Branch CircuitBreakers 134 on L1. In operation, the microprocessor (not shown here)continuously sequences through these switches to measure the voltageacross each breaker. Ground symbol 91 represents the circuit ground ofthe LCI.

In FIG. 10, the voltmeter 137 is connected to the input side of the MainBreaker at 138. It is connected here because this point is easilyaccessible for the magnetic probe on the input lugs of the Main Breaker.Consequently, the voltage measured is across the resistance of both theselected Branch Circuit Breaker 134 and the Main Breaker 108 in series.In a Sub-Panel there is no Main Breaker and so the voltage measured isacross the selected Branch Circuit Breaker 134 alone.

Referring now to FIG. 11, a simplified block diagram of an Input Moduleis shown. Connector 146 (corresponding to connector 207 in FIG. 5) isessentially a feed through to Connector 147 (corresponding to connector208 in FIG. 5). The bus connecting connectors 146 and 147 includes Line1 (L1), Line 2 (L2), Ground (G), DC power for the microprocessor 148 (P)and a multi-drop bi-directional communication line (C). This busstructure allows Input Modules to be daisy-chained as needed toaccommodate the number of breakers in the Load Center. A power supply158 fed by diode 159 from ground 160 develops a DC voltage relative toL2 to power the detection circuits on that phase, comprising aprotection circuit 154, an amplifier/detector 155 and a linearopto-transmitter 156. The output of 156, responsive to the voltageacross the selected breaker, is coupled to the ground referencedmicroprocessor 148 by opto-receiver 157 to provide galvanic isolation.One or more LED's 161 or other indicators may be included to indicatemodule power and/or status conditions.

The small AC voltage developed across each Branch Circuit Breaker issensed by one of the two detection circuits: one whose circuit common isL1 and the other whose circuit common is L2. But each breaker pole mightbe fed by either L1 or L2 depending on its position in the Load Center.The circuitry in the Input Module therefore provides a switching meansto connect each probe to either the L1 or L2 detection circuits.Referring again to FIG. 11, we see a magnetic probe 149, one of a seriesof probes magnetically connected to the terminals of the branch circuitbreakers, as above, which feeds through a current-limiting resistor 150and switch 151 to sensing circuits 154, 155, 156, as above, which arereferenced to L2. The same probe 149 feeds through current-limitingresistor 152 and switch 153 to similar circuits referenced to L1. Thatis, each of the probes, which are individually connected to one of thecircuit breakers, is provided with two switches, corresponding toswitches 151 and 153, with associated current limiting resistors 150,152. On startup both switches 151 and 153 are open; the microprocessormust determine which phase the probe is connected to and thereforedetermine which switch to activate to measure the voltage. If probe 149,for example, is connected to a L1 breaker and switch 151 is closed, afull 240 volts will be seen and current flow will be limited only by thevalue of current-limiting resistor 150. If switch 151 is open and switch153 is closed, the proper arrangement if probe 149 is connected to L1,then only a very small current will flow due to the voltage across thebreaker's internal resistance. One novel aspect of the current circuitis to incorporate an over-current detection means in protection circuit154 that can instantly (e.g. within microseconds) switch off all theswitches when the current exceeds a preset low value, thereby limitingthe power dissipation in current-limiting resistor 150 to an acceptablevalue. The first step during startup, therefore, is to determine, byalternately closing switches 151 and 153 corresponding to the series ofprobes, the phase to which each branch circuit breaker is connected.

In summary then, each Input Module resides on a bus that contains L1,L2, a communications line and ground-referenced DC power for themicroprocessor. Voltage-measuring circuits ride on each of the twophases to measure the AC voltage relative to that phase, and thentransmit the data via a linear isolator to the ground-referencedmicroprocessor, which communicates directly on the bus to othercomponents. The microprocessor is capable of detecting which phase eachinput is referenced to and of switching the input to the appropriatephase. During operation, the microprocessor generally measures thevoltage drop across each breaker sequentially and reports the readingson the communications line.

The preferred embodiment of the current-measuring circuits is shown inthe simplified schematic diagram of FIG. 12. Back-to-back LEDs 163 and164 are the input diodes of two fast opto-isolators that when activatedimmediately shut off all the switches. They also serve to clamp theinput line to within a couple of volts of L2 to protect the amplifiercircuits. In the event that any probe is connected to the wrong phase, arelatively large current (on the order of 50 mA) will flow through oneof the LEDs and thereby quickly open the switches. The microprocessor148 senses this condition and tries the other switch. Since the voltagedeveloped across the breaker due to current flowing through it is lessthan the ON voltage of the LED, the switches will be allowed to remainon only when switched to the right phase. If the probe is connected tothe wrong phase, to ground or if the circuit breaker is switched OFF,the protection circuit will instantly turn off all the switches.

The preferred method of measuring the small voltage developed across thebreaker is by way of a switched integrator 166 configured to integratethe voltage over each positive half-cycle of the line voltage, therebyproviding an average of the current over the half-cycle and goodhigh-frequency noise rejection. Since the PCA applies a rectified load,i.e., current flows through the load only during positive half-cycles,the integrator need only measure the voltage during the same positivehalf-cycles.

Integration is accomplished by employing a switch 168 to alternatelycharge and reset integration capacitor 172 on successive half-cycles ofthe line. The switch 168 is very simply driven by the line voltageitself through resistor 169 and clamped to the positive DC rail by diode170.

FIG. 14 shows a simplified schematic of the Main Module 35 of the LCI15. This module contains a microprocessor 177, which serves as themaster controller for the LCI. The microprocessors in each Input Moduleare there only to control local switching and to minimize the number ofinterconnects needed. Two bus connectors 179 and 180 are provided, onefor each of the Input Modules on each side of the Load Center. A powersupply 181 provides ground-referenced DC power for microprocessor 177and all the Input Modules via the bus line 182 labeled P (Power) and thebus ground 183 labeled G (Ground). This power supply 181 is preferablypowered by both phases so that the LCI can operate with either or bothphases connected. A bi-directional multi-drop communications line 184labeled C (Communications) allows the microprocessor 177 to communicatewith any Input Module. This communications line may consist of one ormore physical wires depending on what signaling method is selected. Anoptional radio link 185 may be included to allow direct radiocommunications with the PDA. It is optional because the umbilical cable17 serves as a direct communications link between the LCI and the PCAwhich itself has a radio link to the PDA. The preferred embodiment ofthe LCI, as illustrated in FIG. 5, also includes LEDs to indicatecommunications activity 209, power on L1 (210) and power on L2 (211).

As noted above, a coaxial umbilical cable 17 connects the PCA to the LCIand serves a number of useful functions. The shield of the umbilicalconnects the PCA circuit ground to the LCI circuit ground 91 to providea source ground reference for PCA measurements. The center conductor ofthe umbilical 89 serves a number of purposes simultaneously. First, theUmbilical Interface block 197 continuously couples DC power (V+) frompower supply 181 to the center conductor to power the PCA. The UmbilicalInterface (UI) block 197 is controlled by the microprocessor 177 viacontrol lines 199 to switch the center conductor 89 as needed to supportother functions. Normally, the center conductor 89 is connected as abi-directional communications line between the PCA and the LCI. Whenneeded, it is switched to couple the distance-measuring pulse to the L1and L2 via line 198.

The LCI and the PCA work together in the SafeWire™ system to perform acomplete inspection of household wiring. The remaining functional blocksof FIG. 14 can only be discussed relative to this inspection so thosetests that apply will be summarized below.

The first basic wiring test measures the resistance of each wire in anoutlet under load. The measured resistance at the PCA is the totalsource resistance seen at the outlet, including both the resistance ofthe wire from the outlet to the Load Center and the resistance leadingup to and including the Load Center. One objective of SafeWire™ testingis to determine the gauge of the wire (if the length of the wire and theresistance of the wire are known, the gauge can be easily estimated), sothere exists a need to separate the resistance of the wire from theoutlet to the Load Center from the resistance leading up to the LoadCenter. This is accomplished by the inclusion of two Multi-FunctionInterface blocks, 118 and 119, fed by current-limiting resistors 117 and116 respectively, that can measure the voltage produced at the input tothe Load Center when the PCA test load is applied. From thesemeasurements, the source resistance feeding the Load Center can becalculated and subtracted from the total resistance measured to obtainthe Branch Circuit wire resistance. The wire resistance from the outletto the Load Center can then be obtained by subtracting the Main andBranch Circuit Breaker resistances, measured in the manner describedbelow.

The second basic wiring test is to determine to which Branch Circuit theoutlet is connected. During PCA testing the load is applied forapproximately one second or 60 line cycles. As described above the LCImeasures the voltage drop across each Branch Circuit Breaker byintegrating one half-cycle and resetting the other half-cycle. Thus ittakes one line cycle to measure the voltage drop across each breaker andthus the voltage drop across sixty breakers can be measured during aone-second PCA load test. Simply put, the Branch Circuit Breaker thatshows a corresponding increase when the PCA load is applied is theBranch Circuit to which the outlet under test is connected.

The third basic wiring test performed by the SafeWire™ system is tomeasure the length of the wire from the outlet to the Load Center usingthe novel method described below, in Section 6. Briefly, when given thecommand, the microprocessor 177 switches the Umbilical Interface 197 toroute the pulse on the umbilical to line 198 which is conditioned bybuffer 200 and then applied to L1 and L2 through the High FrequencyCoupling block 201 via AC coupling capacitors 202 and 203. The rest ofthe length measurement is performed by the PCA, as detailed below.

The fourth and final basic wiring test performed by the SafeWire™ systemis to monitor the line for high-frequency noise at the Load Centerduring various tests. Electrical arcing, even at very low currents,produces wideband high-frequency noise that is conducted down the wiringto the Load Center. The present inventor has a number of patents thatdescribe swept radio-frequency detection means responsive to arcing,including U.S. Pat. Nos. 5,729,145, 5,223,795, 5,434,509, and 5,223,795.The same High-Frequency Coupler (HFC) 201 used for wire lengthmeasurements is used to couple high-frequency signals from L1 and L2 toa radio-frequency arc noise detector. A high-frequency buffer 204 feedsa Radio Frequency Detection (RFD) block 205 controlled by themicroprocessor 177 via control lines 206. The RFD block 205 may be, forexample, a narrowband radio receiver swept by control lines 206. Theoutput of the RFD block 205 is preferably responsive to the logarithm ofthe wideband noise. When arcing is detected, the microprocessor 177measures the number of events and the maximum duration event duration,and sends this data to the PDA via the Bluetooth radio link 185 which,in turn, displays a window indicating to the electrician that arcing hasoccurred.

The arc sensing feature of the LCI can be used to advantage to quantifyarcing that normally does occur, such as when a switch is opened orclosed, and to detect arcing that normally should not occur, such as alamp fixture arcing when the fixture is physically tapped, or arcingthat may occur when the PCA applies a load to the branch circuit. Thenovel method of the present invention is for the electrician to providea stimulus with one hand, in the form of flipping switches, tappingfixtures or running a PCA test, while viewing the PDA in the other handwhich is programmed to display any arcing that may occur. This abilityto view the display while simultaneously providing a stimulus is theprincipal reason why the PDA of the preferred embodiment is adapted tobe wireless.

To test a switch, the electrician carries the PDA in one hand andtoggles the switch with the other. The PDA will display and record thenumber of arcs and the total arc duration, for both the switch ON andthe switch OFF arcs. Several arcs on the order of a few milliseconds induration are typical. Too many arcs or too long a duration or arcingthat persists after the switch is flipped are indicative of a worn-outswitch and a potential fire hazard. Please refer to the presentinventors cited patents above for more discussion of the nature anddetection of arcing. Similarly, any arcing that occurs during PCAtesting will automatically cause a window to pop-up on the PDA thatdisplays the number and duration of the arcs detected. To test lightfixtures and the like, particularly fixtures that normally get hot andare therefore more likely to developing arcing faults, the electriciantaps the fixture with a wooden stick or the like while watching forarcing on the PDA. Finally, appliances can be readily check for bothswitch arcing and arcing due to physical stimulus in a similar manner.

4. Safe Wire™ Magnetic Probe

The SafeWire™ Load Center Interface requires temporary sensingconnections to be made to each branch circuit in a Load Center. Directcontact alligator clips could be used, but the close proximity of linevoltage high-current contacts makes using such clips quite unsafebecause of the real probability of one slipping off or shorting to thechassis or another contact. There exists a need, therefore, for a bettermechanical arrangement for such temporary connections.

The Magnetic Probe means of the present invention is a safe andconvenient means to make temporary electrical connections to ferrouscontacts for testing purposes. The present inventor realized that sincecircuit breakers commonly use steel bolts to secure the wire to acircuit breaker, because non-ferrous materials like brass are too softto support the torque required to secure the connections, a magneticsensing probe could be used to advantage. The idea is to simply stick amagnetic probe on to the commonly flat surface of the bolt to makecontact to the circuit.

FIG. 15 shows a mechanical drawing of a magnetic probe according to thepreferred embodiment of the present invention. The magnet 212 extendsout the end of an insulating handle 213, which is attached to the end ofa sensing wire 214. FIG. 16 shows one such probe 216 attached to atypical Branch Circuit Breaker 215 and a second probe 217 positionednear to but not attached to the circuit breaker wire clamping bolt 218.The size of the magnet and probe are sized to accommodate the bolt 218.FIG. 17 shows another probe 220 attached and nearby but unattached to atypical Main Circuit Breaker (Service Breaker) 219. The large gaugeservice wires are normally secured to the circuit breaker with largesteel socket-head setscrews 221, and the size of the probe 220 isadapted to suit these larger setscrews. FIG. 18 shows yet another probe223, this one adapted to attach to the screw 224 of a typical groundingor neutral bus bar 225.

One probe is needed for each circuit breaker pole in the SafeWire™system. These poles can number up to 48 or more, so it is thereforeadvantageous to have each probe cost as little as possible tomanufacture. FIG. 19 shows the preferred construction of a low costprobe. The magnet 226 is made to press fit into the end of the probebody 228 which is made of an insulating plastic or retained by someother convenient means. The magnet 226 is preferably coated with ahighly conductive, non-corroding metal like nickel or gold. A ferrous,i.e., steel, spacer 227 is adapted to retain the outer insulation of thetest wire 230, by means of inside screw threads or the like or bycrimping the spacer 227 onto the wire 230.

The wire 230 is fed first though retainer 229 and then through spacer227, stripped back to expose the multi-strand conductor 231, and thenflared out as shown. The outer diameter of spacer 227 is small enoughthat it slides freely inside the probe body 228. As the assembly isinserted into the probe body 228, the steel spacer is strongly attractedto the magnet 226, thereby compressing the flared wire strands 231between the magnet 226 and the spacer 227, and making a secure anddirect contact between the conductor strands and the conductive metalcoating on magnet 226. The complete assembly is shown in FIG. 20.

In the SafeWire™ system the test probes are attached to line voltage,high current capacity contacts. Consequently, it is important toincorporate some means of fusible protection in the event that the otherend of the probe wire is inadvertently shorted to ground or the oppositephase. The simplest way to do this is to place a very fine conductor,perhaps on the order of 34 gauge or less, in series between the cable230 and the magnet 226, so that the wire itself will fuse open in theevent of a short circuit. The preferred embodiment of the Magnetic Probeuses a fusible protection means 232 in incorporated in the probe body.This protection means 232 may consist of a fuse or, in someapplications, a high-value resistor which itself limits the current tolow values.

5. AC Balance Method

This section relates to a circuit and method for measuring theresistance of household power distribution wiring under load. In itssimplest embodiment, the circuit is implemented as a stand-alone plug-inmodule that when plugged into a standard 3-prong grounded householdoutlet can determine the individual resistances of each wire feeding theoutlet. This stand-alone unit, however, when plugged into an older2-prong ungrounded outlet or a 2 conductor lamp socket, can determineonly the combined resistance of the two wires feeding the outlet(socket). In the preferred embodiment, therefore, developed for use inthe SafeWire™ system, a separate ground reference wire that runs fromthe Load Center to the outlet (socket) under test, that is, theumbilical cable, is used to enable the method to determine theindividual wire resistances in both 2-wire grounded and 3-wireungrounded outlets.

The basis of this aspect of the present invention, named by the inventorthe AC Balance Method, is to measure the average DC value of the ACvoltage waveform at the outlet (normally very close to zero volts), thenapply a significant rectified load to the outlet and measure the changein average value. By applying the load between different sets of wiresand measuring the difference in DC average values, one can determine theresistance of each wire feeding the load.

Referring first to FIG. 22, a simplified schematic representation of anAC power source, distribution wiring and the stand-alone measurementmeans of the present invention is shown. An AC power source 233 feedsthree wires to distribute power to outlets downstream; a “Hot” wire 234labeled H, a Neutral wire 235 labeled N, and a Ground wire 236 labeledG. Each of these three wires exhibits an inherent length-dependentresistance labeled R_(H), R_(N) and R_(G) respectively. An outlet somedistance downstream has plugged into it a tester according to thepresent invention that comprises a rectifying diode 237, a loadresistance 238 and two switches 239 and 240. By closing switch 239, forexample, the load resistor 238 is placed between wires H and N, whichcauses current to flow through these wires during the positivehalf-cycles of the power waveform. Closing switch 240 causes current toflow through the H and G wires. An average responding voltmeter Vm_(N)measures the average DC voltage between N and G at the outlet andanother voltmeter Vm_(G) measures the average DC voltage between H and Gat the outlet.

First, assume S_(N) is closed and S_(G) open. In this case, currentflows through R_(H) and R_(N) but no current flows through R_(G). WithR_(L) and the line voltage V_(L) known, the current is approximatelyV_(L)/R_(L) and VmN is simply this current times the Neutral resistance.Since current only flows on positive half-cycles, the waveform at Vm_(N)is a half-wave sine wave the average value of which can be simplycalculated by integrating over one complete cycle.

$V_{avg} = {{{\frac{1}{\pi}{\int_{0}^{\pi}{V\;\sin\; x\ {\mathbb{d}x}}}} + {\frac{1}{\pi}{\int_{\pi}^{2\pi}{{0 \cdot \sin}\mspace{11mu} x\ {\mathbb{d}x}}}}} = \frac{2V}{\pi}}$

Thus since, in this case, V=I_(L)R_(N),

${{Vm}_{N}({avg})} = {\frac{2}{\pi}I_{L}R_{N}}$and we see that the average measured voltage is proportional to R_(N).

The Vm_(H) measurement yields the Hot side resistance as follows.Vm_(H)=V_(L)−I_(L)R_(H) during positive half-cycles and Vm_(H)=V_(L)during negative half-cycles. Thus again integrating over one completeline cycle to get the average,

$V_{avg} = {{{\frac{1}{\pi}{\int_{0}^{\pi}{\left( {V_{L} - {I_{L}R_{H}}} \right)\ {\mathbb{d}x}}}} + {\frac{1}{\pi}{\int_{\pi}^{2\pi}{V_{L}\ {\mathbb{d}x}}}}} = {{\frac{1}{\pi}{\int_{0}^{\pi}{V_{L}\ {\mathbb{d}x}}}} + {\frac{1}{\pi}{\int_{\pi}^{2\pi}{V_{L}\ {\mathbb{d}x}}}} - {\frac{1}{\pi}{\int_{0}^{\pi}{I_{L}R_{H}\ {\mathbb{d}x}}}}}}$but since

${{\frac{1}{\pi}{\int_{0}^{\pi}{V_{L}\ {\mathbb{d}x}}}} + {\frac{1}{\pi}{\int_{\pi}^{2\pi}{V_{L}\ {\mathbb{d}x}}}}} = 0$we are left with simply

$V_{avg} = {{- \frac{1}{\pi}}{\int_{0}^{\pi}{I_{L}R_{H}\ {\mathbb{d}x}}}}$

Evaluating this integral we get

${{Vm}_{H}({avg})} = {\frac{- 2}{\pi}I_{L}R_{H}}$

Thus the measured voltage is negative and proportional to R_(H). A novelaspect of this invention is based on the realization that by taking theaverage value of the high-side voltage while applying a rectified load,we are able to reject the high common-mode line voltage (between thepositive and negative half-cycles) and measure the small voltage dropproduced by R_(H) directly. Referring to FIG. 23, one can seegraphically that the average DC value of the sinusoidal line voltage isnormally zero. As illustrated in the graph of FIG. 24, when a rectifiedload is applied, the positive peak alone is diminished thereby drivingthe average value negative, as indicated by the last equation above.

Finally, when S_(G) is closed and S_(N) open, current flows insteadthrough the ground conductor. A similar mathematical analysis yields thefollowing results

${{Vm}_{N}({avg})} = {\frac{- 2}{\pi}I_{L}R_{G}}$

${{Vm}_{H}({avg})} = {\frac{- 2}{\pi}{I_{L}\left( {R_{H} + R_{G}} \right)}}$

Thus, while we can get R_(G) directly, the high-side measurement yieldsonly the sum of R_(H) and R_(G). Since, however, we have alreadymeasured R_(G) we can simply subtract it from the total to get R_(H).

In all the analysis above we make the approximation that

$I_{L} = {\frac{V_{L}}{R_{L}}.}$

More precisely

$I_{L} = {{\frac{V_{L}}{R_{H} + R_{L} + R_{N}}\mspace{14mu}{or}\mspace{14mu} I_{L}} = \frac{V_{L}}{R_{H} + R_{L} + R_{G}}}$depending on which switch is closed. Since the wire resistances R_(H),R_(N) and R_(G) are typically much smaller than the test load resistorR_(L), this approximation will produce a maximum error on the order of afew percent. This will normally be adequate to alert the electrician toa fault. If more precision is required the exact equations can bereadily worked out.

Thus, the stand-alone tester of FIG. 22 plugs into a 3-wire groundedoutlet and can measure each wire's resistance individually by flowingcurrent through one return path while using the other return path (withno current flowing through it and thus no voltage drop across it) toreference the measurement to the voltage source. With older 2-wireungrounded outlets current always flows through both conductors and thusa source reference is not available. As can be seen in the equivalentcircuit of FIG. 25,

${{Vm}_{H}({avg})} = {\frac{- 2}{\pi}{I_{L}\left( {R_{H} + R_{N}} \right)}}$and only the sum R_(H)+R_(N) can be determined. If a fault exists in theR_(N) path, for example, it is desirable to be able to determine theresistances R_(H) and R_(N) independently so that the fault can bequickly located.

To enable individual wire resistance measurements on 2-wire systems aseparate voltage source reference connection can be used as shown inFIG. 26. In this case, the preferred method for the SafeWire™ PCA, aseparate ground reference wire 244 is provided (by the umbilical cablein the SafeWire™ system) to enable all three wires 234, 235, and 236 tobe measured relative to the source voltage ground 244, using techniquessimilar to those described above. Since only a very small amount ofcurrent (microamperes) need flow through resistance R_(m) 245 of theground reference wire 244, the voltage drop across it can be madenegligible. The addition of a third voltmeter, Vm_(G) 243, allows adirect measure of the voltage drop across R_(G) instead of having toderive it as described above relative to FIG. 22. These voltmetersmeasure average dc voltage and can be implemented as simple low-passfilters as described previously in the PCA section.

The AC Balance Method can also detect grounding anomalies such as anopen ground or the ground terminal of an outlet incorrectly wired to theNeutral conductor, or the ground and neutral wires connected together.Referring to FIG. 26, if the ground connection to an outlet is open,Vm_(G) 243 will increase to the average value of the half-wave linevoltage when S_(G) 240 is closed, a level very much greater than if theground connection were made. If the Ground terminal of the outlet isconnected to the Neutral wire of a 2-wire system (no separate ground),then Vm_(G) 243 and Vm_(N) 241 will read precisely the same voltage wheneither S_(N) 239 or S_(G) 240 are closed. If the Ground and Neutralwires on a 3-wire system are connected together at the outlet, thenVm_(G) 243 will show a voltage rise when S_(N) 239 is closed (it shouldnot) and Vm_(N) 241 will show a voltage rise when S_(N) 240 is closed(it should not).

The AC Balance Method disclosed herein can thus be used to advantage toidentify and locate incorrect or deteriorating wiring connections. Ifthe resistance of each wire is known, the wire containing ahigh-resistance fault can readily be identified. A three-wire loadtester can determine the resistances of each wire individuallycalculated in the manner described above. A two-wire load tester canonly determine the sum of the two feedwire resistances unless anexternal ground reference wire is run to the voltage source.

6. Least-Time Propagation Method (LTP)

This section relates to a circuit and method to determine the length ofwiring in household wiring. The methods disclosed herein are intendedfor use in the SafeWire™ Home Wiring Inspection System but can be usedalone as a stand-alone tester. The basic method of the present inventionis referred to by the inventor as the Least-Time Propagation or LTPMethod.

As described in the preceding section, the SafeWire™ Home WiringInspection System employs the present inventor's AC Balance method tomeasure the resistance of feed wires from any outlet to the Load Center.If the length of each wire to the Load Center could also be determinedthen the gauge of the wire, for example, could be calculated, and adetermination made whether the wires were in danger of being overloaded(for example, if a 30-amp breaker were connected to wires only rated for15 amps). The length of the wires to each outlet on a branch circuit, ifknown, could also be used to produce an accurate schematic diagram ofthe circuit. There exists a need, therefore, to provide a method andmeans to determine the length of wires in a house.

Most household wiring uses “Romex”-type cable, which consists of twoinsulated wires, laid parallel to each other in a jacket, with orwithout an uninsulated ground wire in between. The constant physicalspacing of the wires from one another gives Romex cable a fairlyconstant impedance on the order of 75 ohms and makes it capable ofconducting high frequency signals in a transverse transmission linemode. It is a relatively simple matter to measure the length of anisolated section of Romex cable using conventional step time-domainreflectometry (TDR) techniques with good results. Although there is somehigh-frequency dispersion in Romex-type cables, which causes the leadingedge of pulses used for timing to roll-off, lengths of cable up toseveral hundred feet can be readily measured.

However, the present inventor has done extensive experimentation withconventional Time Domain Reflectometry on actual household wiring andfinds that the combination of the less-than-ideal high-frequencycharacteristics of Romex cable, combined with the presence of branchcircuits that split off in the middle of a run to feed other circuits,switch circuits, which cause reflection of pulses, and other factors,render the use of conventional TDR impractical for household wiring. Thenumber of reflections and the low quality of the reflections quicklyrender the reflections nearly indecipherable. There exists a need,therefore, for an improved and simpler technique to measure the lengthsof household wiring.

The present invention provides a circuit and method whereby the lengthof wiring in a household can be readily measured, for example betweentwo outlets or between an outlet and the Load Center, despite thecomplexities imposed by household wiring. The method requires anexternal coaxial cable to be run between the two points being measured.In the SafeWire™ system, this single coaxial “umbilical” cable, which asdiscussed above serves a number of other useful purposes, is spooled outon a retractable reel and generally runs between the Load Center and theoutlet (or lamp socket) under test. Connected in this manner, theinventive LTP method can be used to measure the distance from the outlet(or lamp socket) under test to the Load Center. More generally, however,the LTP method can be used to measure the distance between any twopoints, e.g. any two outlets.

The Least-Time Propagation method is best understood with reference toFIG. 27 which shows the LTP circuit on a 2-wire branch circuit, such asmight be found in older houses. The Load Center, represented as a simplevoltage source 246, feeds a branch circuit (circuit breaker not shown)with Hot conductor 247 and Neutral conductor 248. Outlets 249, 250 and251 are located at various distances from the Load Center. The LTPinstrument, comprising a microprocessor 252 and associated circuitry, isplugged into outlet 251 (it could equivalently be connected to a lampsocket) and a fixed-length coaxial cable 253 runs from the instrument tothe outlet 249.

To measure the distance between outlet 251 and outlet 249, themicroprocessor 252 first drives line driver 263 to produce a pulse atpoint A, the pulse being shown in FIG. 28 (A). The leading edge of thepulse at A propagates down coaxial cable 253 through couplingtransformer 255 and capacitor 256 to produce a pulse at point B. Sincethe length of coaxial cable 253 is fixed, t_(ref) (the propagation timealong the fixed length of the umbilical cable) in FIG. 28 is constant.Now the pulse at point B propagates down the Romex cable until itreaches point C where it couples through coupling capacitor 258 andcoupling transformer 257 to Programmable Threshold Detector 259. Thisfast detector has a programmable threshold set by control line 260 andserves to sense the arrival of the leading edge of the pulse. A fastconstant input integrator 261 serves to convert the time intervalt_(cable) (the propagation time along the circuit to be measured) into aDC voltage that can be fed into an analog input on microprocessor 252.Since t_(ref) is known and constant, the microprocessor 252 can beprogrammed to wait that amount of time after issuing the pulse at Abefore starting the integrator. When the pulse is sensed at the PTD 259,the integrator stops integrating and the value is held until themicroprocessor 252 can read the integrated voltage, which is directlyproportional to the distance from B to C. Any difference between thestart of integration and the time the pulse reaches point B can simplybe subtracted out of the result to arrive at the actual distance.

Coupling transformer 255 serves to transform the impedance of coaxialcable 253 to the impedance of the line at outlet 249 and couplingcapacitor 256 serves to AC couple the transformer to the line. Theimpedance of the line varies according to wire type, which two wires aresensed (e.g., H and G or H and N) and whether the signal is sensed atthe Load Center or not. Most Romex wires are on the order of 75 ohmsbetween adjacent wires and 120 ohms between H and N on Romex with aground. At the Load Center, being the confluence of many Romex wires,the impedance is much lower, typically on the order of 10 ohms. Thus thewinding ratio of the coupling transformer is best optimized to matchimpedances in these different situations.

The reason the method has been coined the Least-Time Propagation methodis that the first pulse that arrives at point C, regardless of whatother reflections may eventually arrive, is the pulse originating atpoint B. For example, another outlet 250 disposed between outlets 249and 251 will conduct the signal out the branch and reflect at the firstdiscontinuity it encounters and travel back to point C. However, it willnecessarily arrive after the leading edge from point B arrives becausethat takes the shortest possible path. If coupling transformer 255 isactually at the Load Center, the pulse at point B will propagate downall the branch circuits and tens or hundreds of reflections will findtheir way back to point C—but all of them will necessarily arrive afterthe leading edge we are interested in. This is one principal idea behindthe present invention—that the principle of least time will insure thatregardless of the number or complexity of reflections that will arriveat the sensing point C, the first to arrive is the one of interest andtherefore detectable.

The second basic idea of this invention is the concept of effectivelyperforming time domain filtering by sampling only in a very shortsynchronized time window, so as enable reliable sampling even in thepresence of random impulse noise. While impulses invariably occuroccasionally on household wiring, from lamp dimmers and the like, oreven remote lightning impulses, the LTP method only looks for an edgeduring a very short interval of time after it issues the leading edgepulse, so as to greatly reduce the chance of one of these random pulsesinterfering with the measurement. The measurement process can also berepeated a number of times to further reduce the chance of interferenceand confirm the results.

More specifically, again referring to FIG. 28, the PTD 259 is enabledonly during a period of time t_(cable) extending from when the leadingedge reaches point B for a period just long enough for the maximumexpected cable length to be measured. With the propagation speed inRomex cable on the order of 1.7 ft/ns, and a maximum cable length of say1000 ft, this amounts to a detection window t_(cable) of only 600 ns.Because of this narrow time-domain detection window, an enormous amountof filtering is afforded which results in high detection reliability.When the LTP method is used in the PCA of the SafeWire™ system, allloads in the house are deliberately turned off, and the system is“quiet” electrically. Pulses do occur but the probability of themoccurring in this extremely narrow time slot are diminishingly small.For this reason, detection reliability is high. Furthermore, as noted,since the entire measurement process is quick, the process can easily berepeated a number of time and the result arrived at only on successiverepeatable results.

Due to less than ideal high-frequency characteristics of Romex cables,the pulse may experience significant attenuation on long lines, as notedabove. The leading edge will also roll off somewhat due tohigh-frequency dispersion effects. To achieve the highest distanceaccuracy, the threshold of the Programmable Threshold Detector 259should be set to the lowest practical level by microprocessor 252. Thiscan be achieved by adaptively setting the threshold to a level justabove the noise floor, i.e., to a point where random thresholdexcursions are just infrequent enough to enable reliable detection ofthe desired edge.

The measurement made on a 3-wire grounded outlet is very much the sameas with the 2-wire system of FIG. 27. Referring now to FIG. 29, we seethat the only substantial difference is that the coupling transformersare now connected between Hot 247 and Ground 266 instead of Hot 247 andNeutral 248. The transmission line impedance between adjacent wires (˜75ohms) is lower than that between conductors separated by a thirdconductor (˜120 ohms). If the connections (outlet 249) are made at theLoad Center there is no difference whether coupling transformer 255 isconnected to Hot and Neutral or Hot and Ground because Neutral andGround are connected together there. Optimal signal response at outlet251, however, is between Hot and Ground as connected in FIG. 29.

As discussed previously, the end of coaxial cable 253 can be pluggedinto an outlet or connected to the Load Center. With the SafeWire™system, it is normally connected to the Load Center through circuitscontained in the Load Center Interface or LCI, thereby measuring thewire length from the outlet (or lamp socket) under test to the LoadCenter. This coupling transformer 255, in this case, is optimized tomatch the coaxial cable impedance (typically 50 ohms) to the Load CenterImpedance (typically 10 ohms).

On occasion, it may be desirable to measure the length of wire betweentwo outlets. To do so with the SafeWire™ system, the umbilical cable 253can be replaced with another cable that plugs into an outlet, so thatthe length of wire between two outlets can be determined. In this case,it may be desirable to be able to detect whether the end of cable 253 isplugged into a socket or not. This can be detected by the modifiedcircuit of FIG. 30. The modification consists of adding a comparator269, which is connected by line 268 to the umbilical cable. A resistor267 plus the output impedance of buffer 263 combine to match thecharacteristic impedance of the umbilical 253. If the couplingtransformer 255 at the end of coaxial cable 253 is floating (that is, isnot connected to the wiring), the pulse will be reflected with noinversion back to point A, the threshold of comparator 269 being set tobe responsive to its increased voltage. When the coupling transformer255 is connected to the line, the reflection there will be minimized andthe higher threshold of comparator 269 will not be reached.

Measuring the reflected wave on umbilical 253 can also be used toautomatically measure the length of the umbilical cable. This may beuseful if extension cables are supplied to lengthen the umbilical. Inthis case, a switch (not shown) operated by microprocessor 252 would beincluded to optionally switch line 268 to feed into programmablethreshold detector 259, the threshold of which is set to trigger on thereflected wave from an unterminated umbilical 253. In this manner, thelength of the umbilical can be measured before it is plugged into theLCI using conventional TDR methods. If it is desired to be able tomeasure the length of umbilical 253 while it is plugged into the LCI, aswitched connection could be added to the LCI to either open or shortthe umbilical 253 so as to provide a reflected waveform. In any case,these operations should require little or no input from the electrician,serving only to automatically compensate for the presence of one or moreextension umbilical cables.

7. Electromagnetic Wire Locating Method (EWL)

This section relates to a circuit and method to detect and locate hiddenhousehold wiring. The methods disclosed herein are intended for use inthe SafeWire™ Home Wiring Inspection System but can also be used in astand-alone device. The basic method of the present invention isreferred to by the inventor as the Electromagnetic Wire-Locating (EWL)method.

During the course of standard SafeWire™ testing, as described in Section2 with reference to FIG. 26 (a portion of which is replicated in FIG.31, using the same reference numbers as FIG. 26) the PCA applies arectified test load to each outlet. Referring now to FIG. 31, theboxed-in portion labeled PCA 273 includes an additional switch 272, thepurpose of which is to direct the load test current from the Hotconductor 234 either back to the outlet 251 through line 271 or througha separate line 270 (i.e., the umbilical cable) back to the voltagesource 233 (typically the Load Center). During standard PCA testing thenew switch 272 is in the position shown and the load current I_(H) flowsdown the Hot conductor 234, through the rectifier 237 and load resistor238 and then back through either the Neutral conductor 235 or the Groundconductor 236 depending on whether switch 239 or switch 240 is closed.In either case, a current I_(L), equal and opposite to I_(H), flows backdown the conductors, causing the magnetic field around the conductors tobe nearly completely cancelled. One potential way to detect and locate ahidden wire is to sense the magnetic field near the wire generated bythe current flowing through it. Unfortunately, when the return currentpath is closely coupled to the source current path, as is the case inRomex wire and the like, the magnetic field is substantially cancelled.By changing switch 272 so that the current flows instead through aseparate return path 270, which is physically distanced from the sourcepath 234, the magnetic field is no longer cancelled and thereforedetection and location of the wire by sensing the magnetic field becomespractical.

The first novel idea of the Electromagnet Wire Locating method of thepresent invention is thus to route the test load current back to theLoad Center through a physically removed path so as to not cancel themagnetic field generated. A convenient path with the SafeWire™ system isthe shield of the SafeWire™ Umbilical cable because it is already routedback to Ground at the Load Center. It is not necessary, however, toroute the current all the way back to the Load Center. All that isneeded is to route it away from the hot source conductor in the areabeing searched to locate the wire. Therefore, another approach would beto route the current externally just as far as another outlet, forexample, as shown in FIG. 32. In this case, the return wire 270 isrouted to the Neutral conductor 235 at an outlet 249 between the outletunder test 251 and the voltage source 233.

To locate a non-energized hidden wire, such as the Ground wire 236 orthe Neutral wire 235, that is, separately from the Hot wire as discussedabove, current must be externally supplied to the wire, so that it willemit a magnetic field that can be detected. Referring now to FIG. 33,another switch 274 is added to the LCI, and serves to switch in a powersupply 275 that provides the current needed to generate a magneticfield. To locate the Ground wire 236, switch 239 is closed and currentI_(L) flows through the Ground wire 235, the current being the voltageof the power supply 275 divided by the resistor R_(PS) 276. The powersupply 275 is preferably derived from the line voltage 234 to providehalf-wave rectified pulsed power to generate a similar pulsed magneticfield.

The preferred embodiment of the EWL tool in the SafeWire™ system makesuse of a two-axis magnetic field sensor to determine not only theproximity of the hidden wire but also the axis the wire lies on. To makeuse of this information a two-dimensional display is needed; in thepreferred embodiment of the SafeWire™ system, the display of the PDA isemployed. The sensor and circuits of the EWL tool, shown in thesimplified block diagram of FIG. 34, are built into a thin “sled”package that attaches to the back of the PDA and mates with the PDA'sinterface connector. Power and communications with the PDA are suppliedthrough this interface. In this manner, the PDA, with the thin sledattached, can be slid across the surface of a wall having hidden wiresbehind it, and will displaying graphics responsive to the sensedmagnetic field.

Referring first to FIG. 36, we see how the tool is used. In thisillustration, the wire 303 is hidden behind a sheetrock wall, forexample. Current I_(L) 304 is flowing through the wire 303 in thedirection indicated. The PDA in a first position 300 displays an arrow302 which indicates the axis of the wire and the direction of thecurrent flow as simply derived from the two-axis magnetic field sensor.The strength of the magnetic field, and thus the proximity of the wire,is indicated by the length of the arrow 302, the arrow being the longestwhen it is directly over the wire, as shown in FIG. 36. As the PDA isrotated, say to a second position 301, the arrow remains stationary. Asthe PDA is moved off-center, to the left for example, the arrow will getshorter as the magnetic field diminishes, but continue to indicate boththe axis of the wire and the direction of the current flow.

The arrow display on the PDA will only be responsive to the wire throughwhich current is being drawn by the PCA. In practical usage, theelectrician might plug the PCA into an outlet and then with the PDAinitiate the wire-locating mode during which the PCA periodicallyapplies the test load, perhaps a few cycles every second, whilesynchronizing the EWL tool to it. The electrician can then slide the PDAalong the surface of the wall to locate the wire feeding the outlet andfollow it upstream towards the Load Center.

Referring now to FIG. 34, a two-axis magnetic field sensor 277 sensesthe magnetic field and provides two outputs 278 and 279 each responsiveto the magnetic field in the direction indicated. A bridge-typemagnetoresistive sensor with a range of 0 to ±6 Gauss, such as theHoneywell HMC1022 works well. Power to the sensor bridge comes frompower supply 280. Each output is amplified in amplifiers 281, detectedwith synchronous detectors 282, and then fed to an analog input inmicroprocessor 284.

Synchronous drive and detection is used to substantially reject themagnetic field resulting from background current and static backgroundfields such as the earth's magnetic field (0.5 Gauss). Backgroundcurrent may be present on the line being traced, if for example, anotherload is present downstream. The basic principal of synchronous detectionis to invert the detected signal during alternate half-cycles of theline voltage and then integrate the result. In this manner, any signalthat persists over two successive half-cycles will integrate to zero andtherefore be substantially rejected. Synchronous drive is inherent inthe half-wave rectified load of the PCA, the load current being ON onlyon positive half-cycles, so this signal will pass through without beingrejected. The microprocessor in the EWL tool 284 drives the synchronousdetectors 282 with a signal on line 283 digitally responsive to when theload is ON. This signal can be obtained from the PCA through theBluetooth radio link 285, preferably using the audio carrier feature ofBluetooth for real-time response. Alternatively, a direct wired linkcould be used. Referring now to FIG. 35, we see the voltage output onone of the axes. The output is the sum of the pulsed current response289 due to current flowing in the wire, and the static earth's magneticfield V_(e) 290. The difference voltage V−V_(e) is responsive to themagnetic field on that axis.

Although the EWL tool of the preferred embodiment uses a two-axismagnetic field sensor, it may be useful to employ a three-axis magneticfield sensor which are also commonly available. For example, the wiresleading to an outlet box may come from directly behind the box, i.e.,perpendicular to the wall. In this case, it may be useful to theelectrician to indicate this. Adapting the preferred embodiment of thepresent invention discussed above to add another axis is easily done bysomeone skilled in the art.

8. Software and System Operation

The SafeWire™ system employs several microcontrollers and anoff-the-shelf PDA to perform a wide variety of tasks as discussed above.Generally, the embedded microprocessors in the PCA, LCI and the μEDTcontrol the hardware, acquire measurements, and transmit the results tothe PDA. The PDA, having much greater computing power than the embeddedmicrocontrollers, then processes and displays the results. The PCA andLCI communicate over the Umbilical cable. The PDA communicates with boththe PCA and the μEDT over a Bluetooth radio link to allow theelectrician to view results while moving about freely.

The SafeWire™ System of the preferred embodiment uses custom softwarewritten for the PDA in C or C++. In the prototype, the software iswritten for the Palm Pilot series of PDAs (or equivalent) using the PalmAPI (Application Program Interface) and running on a Palm operatingsystem, available from Palm, Inc. or PalmSource, Inc. An equivalentsystem could be written for other PDAs or Laptops using differentoperating systems such as the Windows CE or Pocket PC operating systems.

Referring to FIG. 37, comprising five “screen shots” from the PDA,labeled FIGS. 37( a)–(e) respectively, the top-level screen on a PalmPilot PDA is shown in FIG. 37( a). The shaded lower portion of thescreen is common to all Palm applications, providing system buttons(icons) and a writing area for user input. A vertical line of buttons onthe right side allow the electrician to select the basic mode ofoperation from three groups: The “Tester” button switches to a screenthat allows testing of receptacles, including outlets and light sockets,without regard to documenting the location of these devices. NormalSafeWire™ testing, wherein the location of each device is documented, isperformed on the screens accessed by the “Location”, “Outlets”, and“Lights” buttons. As testing proceeds, the loads connected to a switchor a circuit breaker can be reviewed by pressing the “Switches” or“Breakers” buttons.

In the preferred embodiment of the SafeWire™ top-level screen (FIG. 37(a)), a simplified dynamic picture 306 of the system indicates to theelectrician, at a glance, how much of the SafeWire™ system is connected,i.e., each component appears as it is plugged in. On this screen andmost other SafeWire™ screens an indicator 307 flashes to indicatecommunications activity.

The Tester button brings up the Tester screen (FIG. 37( b)), labeled asindicated on the folder tab 309. The screen is initially blank because,as indicated on the status line 311, no adapter plug is yet selected.The electrician selects a PCA adapter plug 6 (FIG. 1) and plugs it intothe PCA cable 3. The PCA 2 automatically senses the type of adapter, asdiscussed in section 2 above, and displays an image of the receptacle,with the name of the receptacle displayed above it, as shown in FIG. 37(c). There are many other types of receptacles, including 2-prongungrounded, GFCI, AFCI, 2 and 3 prong 240 volt power outlets, a varietyof light sockets and so forth, each presenting its own correspondinggraphic and label. When the adapter plug is plugged into a poweredoutlet, the system will automatically sense the presence of voltage andconsequently perform a predetermined series of tests on the outlet anddisplay the results as shown in FIG. 37( d). Note that the status line311 now says “Test Complete” and that individual results are displayedfor each contact of the receptacle, the results displayed in a scrollingbox, i.e., the down arrow can be selected to scroll the display for moretest results. In this example, the results in the upper left box areNeutral (the name of the wire connected to this contact), 1.233V (thevoltage at full load), 2.2% drop (the percentage drop or rise at fullload), 133 ft (the length of this wire back to the Load Center), and 12ga. (the calculated gauge of this wire). The full load current is thebranch circuit breaker rating and the results are calculated byextrapolating from the voltage drop (rise) at the test current load,typically 10 amperes. The additional screen of FIG. 37( e) illustratestypical test results on a 3-prong 240 volt appliance outlet. Pressingthe OK button takes the user back to the top level screen (FIG. 37( a)).

The next group of buttons, i.e., Location, Outlets and Lights, accessthe screens used for full SafeWire™ testing of a house. Referring toFIG. 38( a), the first button brings up the Location screen (FIG. 38(b)) as indicated on the folder tab 309. The Status line 317 now displaysthe last time this location was SafeWire™ tested. This screen simplyaccepts and displays location information, i.e., the name and address ofthe customer, stored in the SafeWire™ database. A line of buttons 320,common to all SafeWire™ screens with more than one record, allows accessto different records. The OK button returns to the top-level screen(FIG. 38( a)). The next two and the last two buttons access the first,previous, next and last records in the database, respectively. Thefourth button displays the record number, in this case 2. By tappingthis button, one can directly input the desired record number to go to.Also, common to most SafeWire™ screens is a comment line 318, used toenter written comments associated with this record into the database,and a microphone icon 319, which when pressed records a spoken commentthat is then likewise stored in the database. On PDAs with a built-incamera, another icon could be added to store pictures with this record.

The Outlet button brings up the Outlet screen (FIG. 38( c)) which allowsthe electrician to document and test outlets in quick succession. Theelectrician enters, by way of a pull-down list, first the floor name 324(and number 325, if desired), then the room name (and number, ifdesired). Next, the outlet number is entered, as determined by anysimple convention such as clockwise from the room entrance, beginningwith 1. The picture of the outlet 328 and the type of outlet 327 areautomatically presented depending on which adapter cable is attached.Next, checkboxes are provided to indicate whether the outlet is a GFCIor AFCI type. If the outlet is switched, then select the switch numberfrom the pull-down list labeled “Sw?” 329, and if a semi-permanent loadis attached, e.g., refrigerator, oven, etc, select that from the “Load:”pull-down list 330. When the adapter plug is plugged into a live outlet,the test proceeds automatically and a summary of the results isdisplayed on a single line 322 above the buttons. Tapping on thereceptacle picture 328 will bring up the Tester screen (FIG. 37( d) or(e)) showing a more detailed display of the test results. The fourthbutton on the Outlet screen displays the outlet test record number, inthis case 0, and displays a Red, Yellow or Green background to indicate,in a stoplight fashion, the status of each record. For example, if theoutlet test revealed a fault, the background is red. If it revealed someparameter of concern but perhaps not quite a fault, it displays yellow.If all the test results were acceptable, it displays a green background.The Lights screen (FIG. 38( d)) works in a similar fashion to theOutlets screen except that it addressed light sockets instead ofoutlets.

Referring now to FIG. 39( a), the Switches and Breakers screens serve toaccess the database from the perspective of a switch or a circuitbreaker. For example, the Switches screen (FIG. 39( b)) shows aparticular 3-way switch, the location and type of switch being displayedon the first three lines 332. The table below shows four lights 333 onthat switch circuit and then another 3-way switch 334 on the circuit.Similarly, the Circuit Breaker screen (FIG. 39( c)) shows the locationand type 336 of the selected circuit breaker and then a table displayingthe outlets 337 on that circuit breaker. Note that this is where therating of each circuit breaker is shown. Although the system can attemptto deduce the breaker rating from the measured series resistance,circuit breakers from different manufacturers vary somewhat and thatapproach is prone to measurement error due to the low series resistancesinvolved. It may therefore be better to simply enter the pole ratinghere during initial setup of the LCI.

The prototype software was written for use with systems employingcircuit breakers but can be simply adapted to also work with oldersystems that employ fuses for circuit protection, by, for example,adding a “Fuses” screen.

One significant advantage of SafeWire™ testing is that enough data iscollected during the normal course of testing all receptacles in a hometo generate both a Load Center label and a schematic diagram of thewiring. The Load Center label which lists the lights, outlets and evenspecific loads on each circuit breaker is easily compiled by the PDAsoftware and can be printed out and attached to the Load Center. Thelabel is preferably printed on a magnetic “paper” that can beconveniently stuck to the metal door of a Load Center. Magnetic “paper”that can be printed on in a standard inkjet printer is readily availablefrom a number of sources.

The generation of a schematic diagram of the household wiring from thedata compiled during SafeWire™ testing is a little more involved and assuch might best be done on a separate computer, one perhaps with aprinter capable of handling larger paper. Household wiring typicallyincludes junction boxes with wires that branch in multiple directions atonce, Hot wires that branch off separately to switches, and a number ofother complications that, as discussed previously, make conventional TDRtesting wholly impractical. The LTP method disclosed in Section 6,however, can easily measure the lengths of each wire even with suchcomplications. Since SafeWire™ testing measures the length of everywire, all that is required to generate a schematic diagram is to sortout the data, a process easily done by a computer algorithm. Anyambiguities that arise, such as whether two outlets that are on the samebranch circuit and each say twenty feet from the Load Center, areconnected to a single Romex cable or two separate Romex cables, can betagged as ambiguous by the algorithm and either displayed as such on theschematic diagram or resolved by further testing. In this example, theLTP method, as discussed in Section 6, can be further used to measurethe distance between the two outlets, thereby resolving the ambiguity.

While a preferred embodiment of the invention has been disclosed indetail, with various modifications and enhancements mentionedspecifically, others will occur to those of skill in the art, and arewithin the scope of the invention. Therefore, the invention should notbe limited by the above exemplary disclosure, but only by the followingclaims.

1. An electrical measuring system for testing an electrical powerdistribution system, the electrical power distribution system comprisinga load center having branch circuit breakers and a ground, the systemcomprising: a first electrical measuring device electrically andphysically connected to the load center, the device being capable ofmonitoring electrical parameters, selected from the group comprisingvoltage and current, at the load center, a second portable electricaldevice being supplied with a plurality of adapter connectors capable ofbeing plugged into various types of receptacles on a branch circuit andcapable of performing tests of said branch circuits by applying testloads across the contacts of the receptacles and measuring voltages onthe contacts with respect to ground, an umbilical conductor electricallyconnecting the first electrical device to the second portable electricaldevice to provide a common reference ground therebetween, and one ormore computing circuits for controlling the tests and saving theresults.
 2. The system of claim 1 wherein the first electrical device iscapable of monitoring current through each branch circuit breakerindependently and identifying the branch circuit breaker through whichthe monitored current is flowing.
 3. The system of claim 2 wherein thefirst electrical device monitors current flowing through each branchcircuit breaker by measuring the voltage across it in synchronizationwith the application of the load by the second portable electricaldevice.
 4. The system of claims 3 wherein a magnetic probe comprising amagnet with a wire electrically connected to said magnet and a seriescurrent-limiting device, the combination encased in an insulatingprotective cover, is attached to a steel-wire retaining screw on eachbranch circuit breaker to measure the voltage across said branch circuitbreaker.
 5. The system of claim 2 wherein the first electrical devicemonitors current flowing through each branch circuit breaker bymeasuring the output of a current sensor, selected from a groupcomprising current transformers and Hall-effect sensors, insynchronization with the application of the load by the second portableelectrical device.
 6. The system of claim 1 wherein the secondelectrical device is adapted to automatically identify the type ofadapter cable used.
 7. The system of claim 1 wherein the second portabledevice is capable of applying test loads between one of the conductorsof the branch circuit and ground, with a return path through theumbilical conductor, to produce a non-canceling magnetic field aroundthe branch conductor for the purpose of tracing said branch conductor.8. The system of claim 7 wherein a third portable electrical device isprovided to indicate the proximity of, and the direction of current flowin, the branch conductor, the device comprising, a two-axis magneticfield sensor producing two signals, each responsive to the strength anddirection of the magnetic field in one orthogonal direction, anelectrical circuit capable of detecting the signals in synchronizationwith the application of the load by the second portable electricaldevice, and a display device capable of indicating the strength anddirection of the magnetic field.
 9. The system of claim 7 wherein athird portable electrical device is provided to indicate the proximityof, and the direction of current flow in, the branch conductor, thedevice comprising, a three-axis magnetic field sensor producing threesignals, each responsive to the strength and direction of the magneticfield in one orthogonal direction, an electrical circuit capable ofdetecting the signals in synchronization with the application of theload by the second portable electrical device, and a display devicecapable of indicating the strength and direction of the magnetic field.10. The system of claim 1 wherein the first electrical device, uponbeing plugged into a receptacle on a branch circuit, is adapted to sensethe presence of voltage and in response initiate further measurements tobe taken.
 11. The system of claim 1 further including a portablecomputer programmed so as to allow an electrician to exercise control ofthe system, by selecting the tests to be performed, and to view theresults of the measurements and calculations performed accordingly. 12.The system of claim 11 wherein the portable computer is wirelesslylinked to the system via a radio transceiver.
 13. A method enabling anelectrician to test an electrical power distribution system, theelectrical power distribution system having a load center and branchcircuits with receptacles, the method comprising the steps of: attachinga first electrical device to the load center, the device being capableof monitoring electrical parameters in the load center, electricallyconnecting an umbilical conductor between the first electrical deviceand a second portable electrical device, said second portable devicebeing supplied with a plurality of adapter connectors capable of beingplugged into various types of receptacles on a branch circuit, and beingcapable of performing various electrical tests, such that the umbilicalconductor provides a common reference between the first electricaldevice and the second portable electrical device, moving about thebranch circuits and plugging the second portable electrical devicesequentially into receptacles, the device automatically performingvarious electrical tests at each receptacle, and evaluating thecondition of the branch circuit responsive to said tests.
 14. The methodof claim 13, wherein the first electrical device monitors current flowthrough the load center in synchronization with the second portableelectrical device applying a test load between contacts in thereceptacles.
 15. The method of claim 14, wherein the test load isapplied for a period of time during which the first electrical devicemonitors the current in each branch circuit breaker successively,thereby determining the circuit breaker through which the test loadcurrent is flowing.
 16. The method of claim 15, wherein the currentflowing through each circuit breaker is measured by measuring thevoltage across the circuit breaker.
 17. The method of claim 16, whereincontact is made with each circuit breaker, to measure the voltage acrossit, by using a magnetic probe that sticks to a wire-retaining steelscrew in the circuit breaker.
 18. The method of claim 13 wherein aportable computer is programmed so as to allow an electrician toexercise control of the system, by selecting the tests to be performed,and to view the results of the measurements and calculations performedaccordingly.
 19. The method of claim 18 wherein the portable computeruses the measurements to calculate the length and resistance of eachwire tested, from the point of test to the load center, and from theseparameters estimate the gauge of the wire.
 20. The method of claim 18wherein the portable computer uses the measurements and calculations toconstruct a label for the load center, the label listing the number,type and location of receptacles and lights on each branch circuit. 21.The method of claim 18 wherein the portable computer uses themeasurements and calculations to construct a schematic diagram of theelectrical power distribution system.
 22. The method of claim 18 whereinthe portable computer saves the measurements into data tables andrecords these tables onto a non-volatile medium.
 23. The method of claim13 further comprising circuits for monitoring high-frequency noisecharacteristic of electrical arcing while the electrician provides astimulus to generate arcing, selected from the group of switchingswitches, physically tapping fixtures, turning ON or OFF appliances, orconducting load tests.
 24. The method of claim 13, wherein the secondportable electrical device applies a test load between a branchconductor at the receptacle and ground at the load center, the returnpath being through the umbilical conductor, to produce a non-cancelingmagnetic field around the branch conductor for the purpose of tracingsaid branch conductor.
 25. The method of claim 24, wherein theelectrician uses a third portable electrical device, the device beingcapable of sensing the strength and direction of the magnetic field insynchronization with the application of the test load, to locate thebranch conductor.
 26. The method of claim 25, wherein the portableelectrical device senses the magnitude and direction of the magneticfield in two dimensions and provides a display responsive to thedirection for the purpose of locating the conductor.
 27. The method ofclaim 25, wherein the portable electrical device senses the magnitudeand direction of the magnetic field in three dimensions and provides adisplay responsive to the direction for the purpose of locating theconductor.